2008-01-16 21:30:43

by Valerie Henson

[permalink] [raw]
Subject: [RFC] Parallelize IO for e2fsck

Hi y'all,

This is a request for comments on the rewrite of the e2fsck IO
parallelization patches I sent out a few months ago. The mechanism is
totally different. Previously IO was parallelized by issuing IOs from
multiple threads; now a single thread issues fadvise(WILLNEED) and
then uses read() to complete the IO.

Single disk performance doesn't change, but elapsed time drops by
about 50% on a big RAID-5 box. Passes 1 and 2 are parallelized. Pass
5 is left as an exercise for the reader.

Many thanks to the Lustre folks for their fadvise readahead patch
which this patch uses and for comments and help in general. Our good
friends at EMC Centera funded this work.

Here are the top things I'd like feedback on:

How to split up the patch? My take:

* Indirect block only flag for iterate
* IO manager readahead/release functions
* Readahead infrastructure (readahead.c and related)
* Readahead calls for pass 1
* Readahead calls for pass 2

Killing readahead properly is hard. I implemented it several ways and
didn't like any of them. The current solution is still racy and
completely untested.

The whole thing needs to be autoconfed correctly. Bah.

The user interface kinda sucks. It should at least take arguments of
the form "128KB" or "52m" instead of number of file system blocks.
Guessing the right amount of buffer cache to use and io requests to
issue would also be good.

ext2fs_get_next_inode_ptr() - With readahead, copying the inode in
ext2fs_get_next_inode_full() costs about 2-3% of elapsed time. This
is a hacked up version that just returns a pointer to the inode.

The patch is against e2fsprogs 1.40.4 and is attached. Future patches
will be split up and sent via quilt.

Thanks!

-VAL

e2fsck/Makefile.in | 6
e2fsck/e2fsck.h | 5
e2fsck/pass1.c | 36
e2fsck/pass2.c | 12
e2fsck/unix.c | 18
lib/ext2fs/readahead.c | 1607 ++++++++++++++++++++++++++++++++
lib/ext2fs/Makefile.in | 1
lib/ext2fs/block.c | 20
lib/ext2fs/dosio.c | 2
lib/ext2fs/ext2_io.h | 10
lib/ext2fs/ext2fs.h | 42
lib/ext2fs/inode.c | 70 +
lib/ext2fs/inode_io.c | 2
lib/ext2fs/io_manager.c | 12
lib/ext2fs/nt_io.c | 2
lib/ext2fs/test_io.c | 2
lib/ext2fs/unix_io.c | 41
17 files changed, 1873 insertions(+), 15 deletions(-)


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2008-01-18 01:16:03

by David Chinner

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Wed, Jan 16, 2008 at 01:30:43PM -0800, Valerie Henson wrote:
> Hi y'all,
>
> This is a request for comments on the rewrite of the e2fsck IO
> parallelization patches I sent out a few months ago. The mechanism is
> totally different. Previously IO was parallelized by issuing IOs from
> multiple threads; now a single thread issues fadvise(WILLNEED) and
> then uses read() to complete the IO.

Interesting.

We ultimately rejected a similar patch to xfs_repair (pre-population
the kernel block device cache) mainly because of low memory
performance issues and it doesn't really enable you to do anything
particularly smart with optimising I/O patterns for larger, high
performance RAID arrays.

The low memory problems were particularly bad; the readahead
thrashing cause a slowdown of 2-3x compared to the baseline and
often it was due to the repair process requiring all of memory
to cache stuff it would need later. IIRC, multi-terabyte ext3
filesystems have similar memory usage problems to XFS, so there's
a good chance that this patch will see the same sorts of issues.

> Single disk performance doesn't change, but elapsed time drops by
> about 50% on a big RAID-5 box. Passes 1 and 2 are parallelized. Pass
> 5 is left as an exercise for the reader.

Promising results, though....

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group

2008-01-18 01:43:37

by Valerie Henson

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Jan 17, 2008 5:15 PM, David Chinner <[email protected]> wrote:
> On Wed, Jan 16, 2008 at 01:30:43PM -0800, Valerie Henson wrote:
> > Hi y'all,
> >
> > This is a request for comments on the rewrite of the e2fsck IO
> > parallelization patches I sent out a few months ago. The mechanism is
> > totally different. Previously IO was parallelized by issuing IOs from
> > multiple threads; now a single thread issues fadvise(WILLNEED) and
> > then uses read() to complete the IO.
>
> Interesting.
>
> We ultimately rejected a similar patch to xfs_repair (pre-population
> the kernel block device cache) mainly because of low memory
> performance issues and it doesn't really enable you to do anything
> particularly smart with optimising I/O patterns for larger, high
> performance RAID arrays.
>
> The low memory problems were particularly bad; the readahead
> thrashing cause a slowdown of 2-3x compared to the baseline and
> often it was due to the repair process requiring all of memory
> to cache stuff it would need later. IIRC, multi-terabyte ext3
> filesystems have similar memory usage problems to XFS, so there's
> a good chance that this patch will see the same sorts of issues.

That was one of my first concerns - how to avoid overflowing memory?
Whenever I screw it up on e2fsck, it does go, oh, 2 times slower due
to the minor detail of every single block being read from disk twice.
:)

I have a partial solution that sort of blindly manages the buffer
cache. First, the user passes e2fsck a parameter saying how much
memory is available as buffer cache. The readahead thread reads
things in and immediately throws them away so they are only in buffer
cache (no double-caching). Then readahead and e2fsck work together so
that readahead only reads in new blocks when the main thread is done
with earlier blocks. The already-used blocks get kicked out of buffer
cache to make room for the new ones.

What would be nice is to take into account the current total memory
usage of the whole fsck process and factor that in. I don't think it
would be hard to add to the existing cache management framework.
Thoughts?

> Promising results, though....

Thanks! It's solving a rather simpler problem than XFS check/repair. :)

-VAL

2008-01-21 23:00:41

by Andreas Dilger

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Jan 16, 2008 13:30 -0800, Valerie Henson wrote:
> I have a partial solution that sort of blindly manages the buffer
> cache. First, the user passes e2fsck a parameter saying how much
> memory is available as buffer cache. The readahead thread reads
> things in and immediately throws them away so they are only in buffer
> cache (no double-caching). Then readahead and e2fsck work together so
> that readahead only reads in new blocks when the main thread is done
> with earlier blocks. The already-used blocks get kicked out of buffer
> cache to make room for the new ones.
>
> What would be nice is to take into account the current total memory
> usage of the whole fsck process and factor that in. I don't think it
> would be hard to add to the existing cache management framework.
> Thoughts?

I discussed this with Ted at one point also. This is a generic problem,
not just for readahead, because "fsck" can run multiple e2fsck in parallel
and in case of many large filesystems on a single node this can cause
memory usage problems also.

What I was proposing is that "fsck.{fstype}" be modified to return an
estimated minimum amount of memory needed, and some "desired" amount of
memory (i.e. readahead) to fsck the filesystem, using some parameter like
"fsck.{fstype} --report-memory-needed /dev/XXX". If this does not
return the output in the expected format, or returns an error then fsck
will assume some amount of memory based on the device size and continue
as it does today.

If the fsck.{fstype} does understand this parameter, then fsck makes a
decision based on devices, parallelism, total RAM (less some amount to
avoid thrashing), then it can call the individual fsck commands with
"--maximum-memory MMM /dev/XXX" so each knows how much cache it can
allocate. This parameter can also be specified by the user if running
e2fsck directly.

I haven't looked through your patch yet, but I hope to get to it soon.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.


2008-01-22 03:39:14

by David Chinner

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Mon, Jan 21, 2008 at 04:00:41PM -0700, Andreas Dilger wrote:
> On Jan 16, 2008 13:30 -0800, Valerie Henson wrote:
> > I have a partial solution that sort of blindly manages the buffer
> > cache. First, the user passes e2fsck a parameter saying how much
> > memory is available as buffer cache. The readahead thread reads
> > things in and immediately throws them away so they are only in buffer
> > cache (no double-caching). Then readahead and e2fsck work together so
> > that readahead only reads in new blocks when the main thread is done
> > with earlier blocks. The already-used blocks get kicked out of buffer
> > cache to make room for the new ones.
> >
> > What would be nice is to take into account the current total memory
> > usage of the whole fsck process and factor that in. I don't think it
> > would be hard to add to the existing cache management framework.
> > Thoughts?
>
> I discussed this with Ted at one point also. This is a generic problem,
> not just for readahead, because "fsck" can run multiple e2fsck in parallel
> and in case of many large filesystems on a single node this can cause
> memory usage problems also.
>
> What I was proposing is that "fsck.{fstype}" be modified to return an
> estimated minimum amount of memory needed, and some "desired" amount of
> memory (i.e. readahead) to fsck the filesystem, using some parameter like
> "fsck.{fstype} --report-memory-needed /dev/XXX". If this does not
> return the output in the expected format, or returns an error then fsck
> will assume some amount of memory based on the device size and continue
> as it does today.

And while fsck is running, some other program runs that uses
memory and blows your carefully calculated paramters to smithereens?

I think there is a clear need for applications to be able to
register a callback from the kernel to indicate that the machine as
a whole is running out of memory and that the application should
trim it's caches to reduce memory utilisation.

Perhaps instead of swapping immediately, a SIGLOWMEM could be sent
to a processes that aren't masking the signal followed by a short
grace period to allow the processes to free up some memory before
swapping out pages from that process?

With this sort of feedback, the fsck process can scale back it's
readahead and remove cached info that is not critical to what it
is currently doing and thereby prevent readahead thrashing as
memory usage of the fsck process itself grows.

Another example where this could be useful is to tell browsers to
release some of their cache rather than having the VM swap it out.

IMO, a scheme like this will be far more reliable than trying to
guess what the optimal settings are going to be over the whole
lifetime of a process....

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group

2008-01-22 04:17:45

by Valdis Klētnieks

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Tue, 22 Jan 2008 14:38:30 +1100, David Chinner said:

> Perhaps instead of swapping immediately, a SIGLOWMEM could be sent
> to a processes that aren't masking the signal followed by a short
> grace period to allow the processes to free up some memory before
> swapping out pages from that process?

AIX had SIGDANGER some 15 years ago. Admittedly, that was sent when
the system was about to hit OOM, not when it was about to start swapping.

I suspect both approaches have their merits...


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2008-01-22 07:00:50

by Andreas Dilger

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Jan 21, 2008 23:17 -0500, [email protected] wrote:
> On Tue, 22 Jan 2008 14:38:30 +1100, David Chinner said:
> > Perhaps instead of swapping immediately, a SIGLOWMEM could be sent
> > to a processes that aren't masking the signal followed by a short
> > grace period to allow the processes to free up some memory before
> > swapping out pages from that process?
>
> AIX had SIGDANGER some 15 years ago. Admittedly, that was sent when
> the system was about to hit OOM, not when it was about to start swapping.

I'd tried to advocate SIGDANGER some years ago as well, but none of
the kernel maintainers were interested. It definitely makes sense
to have some sort of mechanism like this. At the time I first brought
it up it was in conjunction with Netscape using too much cache on some
system, but it would be just as useful for all kinds of other memory-
hungry applications.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.


2008-01-22 07:05:11

by Andreas Dilger

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Jan 22, 2008 14:38 +1100, David Chinner wrote:
> On Mon, Jan 21, 2008 at 04:00:41PM -0700, Andreas Dilger wrote:
> > I discussed this with Ted at one point also. This is a generic problem,
> > not just for readahead, because "fsck" can run multiple e2fsck in parallel
> > and in case of many large filesystems on a single node this can cause
> > memory usage problems also.
> >
> > What I was proposing is that "fsck.{fstype}" be modified to return an
> > estimated minimum amount of memory needed, and some "desired" amount of
> > memory (i.e. readahead) to fsck the filesystem, using some parameter like
> > "fsck.{fstype} --report-memory-needed /dev/XXX". If this does not
> > return the output in the expected format, or returns an error then fsck
> > will assume some amount of memory based on the device size and continue
> > as it does today.
>
> And while fsck is running, some other program runs that uses
> memory and blows your carefully calculated paramters to smithereens?

Well, fsck has a rather restricted working environment, because it is
run before most other processes start (i.e. single-user mode). For fsck
initiated by an admin in other runlevels the admin would need to specify
the upper limit of memory usage. My proposal was only for the single-user
fsck at boot time.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.


2008-01-22 08:17:05

by David Chinner

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Tue, Jan 22, 2008 at 12:05:11AM -0700, Andreas Dilger wrote:
> On Jan 22, 2008 14:38 +1100, David Chinner wrote:
> > On Mon, Jan 21, 2008 at 04:00:41PM -0700, Andreas Dilger wrote:
> > > I discussed this with Ted at one point also. This is a generic problem,
> > > not just for readahead, because "fsck" can run multiple e2fsck in parallel
> > > and in case of many large filesystems on a single node this can cause
> > > memory usage problems also.
> > >
> > > What I was proposing is that "fsck.{fstype}" be modified to return an
> > > estimated minimum amount of memory needed, and some "desired" amount of
> > > memory (i.e. readahead) to fsck the filesystem, using some parameter like
> > > "fsck.{fstype} --report-memory-needed /dev/XXX". If this does not
> > > return the output in the expected format, or returns an error then fsck
> > > will assume some amount of memory based on the device size and continue
> > > as it does today.
> >
> > And while fsck is running, some other program runs that uses
> > memory and blows your carefully calculated paramters to smithereens?
>
> Well, fsck has a rather restricted working environment, because it is
> run before most other processes start (i.e. single-user mode). For fsck
> initiated by an admin in other runlevels the admin would need to specify
> the upper limit of memory usage. My proposal was only for the single-user
> fsck at boot time.

The simple case. ;)

Because XFS has shutdown features, it's not uncommon to hear about
people running xfs_repair on an otherwise live system. e.g. XFS
detects a corrupted block, shuts down the filesystem, the admin
unmounts it, runs xfs_repair, puts it back online. meanwhile, all
the other filesystems and users continue unaffected. In this use
case, getting feedback about memory usage is, IMO, very worthwhile.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group

2008-01-22 13:09:14

by Alan Cox

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

> I'd tried to advocate SIGDANGER some years ago as well, but none of
> the kernel maintainers were interested. It definitely makes sense
> to have some sort of mechanism like this. At the time I first brought
> it up it was in conjunction with Netscape using too much cache on some
> system, but it would be just as useful for all kinds of other memory-
> hungry applications.

There is an early thread for a /proc file which you can add to your
poll() set and it will wake people when memory is low. Very elegant and
if async support is added it will also give you the signal variant for
free.

Alan

2008-01-22 17:42:54

by Bryan Henderson

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

>I think there is a clear need for applications to be able to
>register a callback from the kernel to indicate that the machine as
>a whole is running out of memory and that the application should
>trim it's caches to reduce memory utilisation.
>
>Perhaps instead of swapping immediately, a SIGLOWMEM could be sent ...

The problem with that approach is that the Fsck process doesn't know how
its need for memory compares with other process' need for memory. How
much memory should it give up? Maybe it should just quit altogether if
other processes are in danger of deadlocking. Or maybe it's best for it
to keep all its memory and let some other frivolous process give up its
memory instead.

It's the OS's job to have a view of the entire system and make resource
allocation decisions.

If it's just a matter of the application choosing a better page frame to
vacate than what the kernel would have taken, (which is more a matter of
self-interest than resource allocation), then Fsck can do that more
directly by just monitoring its own page fault rate. If it's high, then
it's using more real memory than the kernel thinks it's entitled to and it
can reduce its memory footprint to improve its speed. It can even check
whether an access to readahead data caused a page fault; if so, it knows
reading ahead is actually making things worse and therefore reduce
readahead until the page faults stop happening.

--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems

2008-01-24 17:32:26

by Bodo Eggert

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

Alan Cox <[email protected]> wrote:

>> I'd tried to advocate SIGDANGER some years ago as well, but none of
>> the kernel maintainers were interested. It definitely makes sense
>> to have some sort of mechanism like this. At the time I first brought
>> it up it was in conjunction with Netscape using too much cache on some
>> system, but it would be just as useful for all kinds of other memory-
>> hungry applications.
>
> There is an early thread for a /proc file which you can add to your
> poll() set and it will wake people when memory is low. Very elegant and
> if async support is added it will also give you the signal variant for
> free.

IMO you'll need a userspace daemon. The kernel does only know about the
amount of memory available / recommended for a system (or container),
while the user knows which program's cache is most precious today.

(Off cause the userspace daemon will in turn need the /proc file.)

I think a single, system-wide signal is the second-to worst solution: All
applications (or the wrong one, if you select one) would free their caches
and start to crawl, and either stay in this state or slowly increase their
caches again until they get signaled again. And the signal would either
come too early or too late. The userspace daemon could collect the weighted
demand of memory from all applications and tell them how much to use.

2008-01-24 22:08:39

by Andreas Dilger

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Jan 24, 2008 18:32 +0100, Bodo Eggert wrote:
> I think a single, system-wide signal is the second-to worst solution: All
> applications (or the wrong one, if you select one) would free their caches
> and start to crawl, and either stay in this state or slowly increase their
> caches again until they get signaled again. And the signal would either
> come too early or too late. The userspace daemon could collect the weighted
> demand of memory from all applications and tell them how much to use.

Well, sending a few signals (maybe to the top 5 processes in the OOM killer
list) is still a LOT better than OOM-killing them without warning... That
way important system processes could be taught to understand SIGDANGER and
maybe do something about it instead of being killed, and if Firefox and
other memory hungry processes flush some of their cache it is not fatal.

I wouldn't think that SIGDANGER means "free all of your cache", since the
memory usage clearly wasn't a problem a few seconds previously, so as
an application writer I'd code it as "flush the oldest 10% of my cache"
or similar, and the kernel could send SIGDANGER again (or kill the real
offender) if the memory usage again becomes an issue.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

2008-01-24 23:09:43

by Adrian Bunk

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Thu, Jan 24, 2008 at 06:32:15PM +0100, Bodo Eggert wrote:
> Alan Cox <[email protected]> wrote:
>
> >> I'd tried to advocate SIGDANGER some years ago as well, but none of
> >> the kernel maintainers were interested. It definitely makes sense
> >> to have some sort of mechanism like this. At the time I first brought
> >> it up it was in conjunction with Netscape using too much cache on some
> >> system, but it would be just as useful for all kinds of other memory-
> >> hungry applications.
> >
> > There is an early thread for a /proc file which you can add to your
> > poll() set and it will wake people when memory is low. Very elegant and
> > if async support is added it will also give you the signal variant for
> > free.
>
> IMO you'll need a userspace daemon. The kernel does only know about the
> amount of memory available / recommended for a system (or container),
> while the user knows which program's cache is most precious today.
>
> (Off cause the userspace daemon will in turn need the /proc file.)
>
> I think a single, system-wide signal is the second-to worst solution: All
> applications (or the wrong one, if you select one) would free their caches
> and start to crawl, and either stay in this state or slowly increase their
> caches again until they get signaled again. And the signal would either
> come too early or too late. The userspace daemon could collect the weighted
> demand of memory from all applications and tell them how much to use.

I don't think that's something that would require finetuning on a
per-application basis - the kernel should tell all applications once to
reduce memory consumption and write a fat warning to the logs (which
will on well-maintained systems be mailed to the admin).

Your "and tell them how much to use" wouldn't work for most applications
- e.g. I've worked the last weeks with a computer with 512 MB RAM and no
Swap, which means usually only 200 MB of free RAM. I've gotten quite
used to git aborting with "fatal: Out of memory, malloc failed" when
200 MB weren't enough for git, and I don't think there is any reasonable
way for git to reduce the memory usage while continuing to run.

In practice, there is a small number of programs that are both the
common memory hogs and should be able to reduce their memory consumption
by 10% or 20% without big problems when requested (e.g. Java VMs,
Firefox and databases come into my mind).

And from a performance point of view letting applications voluntarily
free some memory is better even than starting to swap.

cu
Adrian

--

"Is there not promise of rain?" Ling Tan asked suddenly out
of the darkness. There had been need of rain for many days.
"Only a promise," Lao Er said.
Pearl S. Buck - Dragon Seed

2008-01-24 23:41:44

by Theodore Ts'o

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Fri, Jan 25, 2008 at 01:08:09AM +0200, Adrian Bunk wrote:
> In practice, there is a small number of programs that are both the
> common memory hogs and should be able to reduce their memory consumption
> by 10% or 20% without big problems when requested (e.g. Java VMs,
> Firefox and databases come into my mind).

I agree, it's only a few processes where this makes sense. But for
those that do, it would be useful if they could register with the
kernel that would like to know, (just before the system starts
ejecting cached data, just before swapping, etc.) and at what
frequency. And presumably, if the kernel notices that a process is
responding to such requests with memory actually getting released back
to the system, that process could get "rewarded" by having the OOM
killer less likely to target that particular thread.

AIX basically did this with SIGDANGER (the signal is ignored by
default), except there wasn't the ability for the process to tell the
kernel at what level of memory pressure before it should start getting
notified, and there was no way for the kernel to tell how bad the
memory pressure actually was. On the other hand, it was a relatively
simple design.

In practice very few processes would indeed pay attention to
SIGDANGER, so I think you're quite right there.

> And from a performance point of view letting applications voluntarily
> free some memory is better even than starting to swap.

Absolutely.

- Ted

2008-01-25 00:25:42

by Zan Lynx

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck


On Thu, 2008-01-24 at 18:40 -0500, Theodore Tso wrote:
> On Fri, Jan 25, 2008 at 01:08:09AM +0200, Adrian Bunk wrote:
> > In practice, there is a small number of programs that are both the
> > common memory hogs and should be able to reduce their memory consumption
> > by 10% or 20% without big problems when requested (e.g. Java VMs,
> > Firefox and databases come into my mind).
>
> I agree, it's only a few processes where this makes sense. But for
> those that do, it would be useful if they could register with the
> kernel that would like to know, (just before the system starts
> ejecting cached data, just before swapping, etc.) and at what
> frequency. And presumably, if the kernel notices that a process is
> responding to such requests with memory actually getting released back
> to the system, that process could get "rewarded" by having the OOM
> killer less likely to target that particular thread.

Have y'all been following the /dev/mem_notify patches?
http://article.gmane.org/gmane.linux.kernel/628653

--
Zan Lynx <[email protected]>


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2008-01-25 18:03:34

by Bryan Henderson

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

> AIX basically did this with SIGDANGER (the signal is ignored by
> default), except there wasn't the ability for the process to tell the
> kernel at what level of memory pressure before it should start getting
> notified, and there was no way for the kernel to tell how bad the
> memory pressure actually was. On the other hand, it was a relatively
> simple design.

AIX does provide a system call to find out how much paging backing store
space is available and the thresholds set by the system administrator.
Running out of paging space is the only memory pressure AIX is concerned
about. While I think having processes make memory usage decisions based
on that is a shoddy way to manage system resources, that's what it is
intended for.

Incidentally, some context for the AIX approach to the OOM problem: a
process may exclude itself from OOM vulnerability altogether. It places
itself in "early allocation" mode, which means at the time it creates
virtual memory, it reserves enough backing store for the worst case. The
memory manager does not send such a process the SIGDANGER signal or
terminate it when it runs out of paging space. Before c. 2000, this was
the only mode. Now the default is late allocation mode, which is similar
to Linux.

--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems


2008-01-25 20:23:59

by Andreas Dilger

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Jan 24, 2008 17:25 -0700, Zan Lynx wrote:
> Have y'all been following the /dev/mem_notify patches?
> http://article.gmane.org/gmane.linux.kernel/628653

Having the notification be via poll() is a very restrictive processing
model. Having the notification be via a signal means that any kind of
process (and not just those that are event loop driven) can register
a callback at some arbitrary point in the code and be notified. I
don't object to the poll() interface, but it would be good to have a
signal mechanism also.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

2008-01-25 23:01:20

by Bodo Eggert

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Fri, 25 Jan 2008, Bryan Henderson wrote:

> > AIX basically did this with SIGDANGER (the signal is ignored by
> > default), except there wasn't the ability for the process to tell the
> > kernel at what level of memory pressure before it should start getting
> > notified, and there was no way for the kernel to tell how bad the
> > memory pressure actually was. On the other hand, it was a relatively
> > simple design.
>
> AIX does provide a system call to find out how much paging backing store
> space is available and the thresholds set by the system administrator.
> Running out of paging space is the only memory pressure AIX is concerned
> about. While I think having processes make memory usage decisions based
> on that is a shoddy way to manage system resources, that's what it is
> intended for.

If you start partitioning the system into virtual servers (or something
similar), being close to swapping may be somebody else's problem.
(They shouldn't have exceeded their guaranteed memory limit).


> Incidentally, some context for the AIX approach to the OOM problem: a
> process may exclude itself from OOM vulnerability altogether. It places
> itself in "early allocation" mode, which means at the time it creates
> virtual memory, it reserves enough backing store for the worst case. The
> memory manager does not send such a process the SIGDANGER signal or
> terminate it when it runs out of paging space. Before c. 2000, this was
> the only mode. Now the default is late allocation mode, which is similar
> to Linux.

This is an interesting approach. It feels like some programs might be
interested in choosing this mode instead of risking OOM.
--
The programmer's National Anthem is 'AAAAAAAAHHHHHHHH'

2008-01-26 00:56:04

by Zan Lynx

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck


On Fri, 2008-01-25 at 04:09 -0700, Andreas Dilger wrote:
> On Jan 24, 2008 17:25 -0700, Zan Lynx wrote:
> > Have y'all been following the /dev/mem_notify patches?
> > http://article.gmane.org/gmane.linux.kernel/628653
>
> Having the notification be via poll() is a very restrictive processing
> model. Having the notification be via a signal means that any kind of
> process (and not just those that are event loop driven) can register
> a callback at some arbitrary point in the code and be notified. I
> don't object to the poll() interface, but it would be good to have a
> signal mechanism also.

The commentary on the mem_notify threads claimed that the signal is
easily provided by setting up the file handle for SIGIO.

Yeah. Here it is...copied from email written by KOSAKI Motohiro:

implement FASYNC capability to /dev/mem_notify.

<usage example>
fd = open("/dev/mem_notify", O_RDONLY);

fcntl(fd, F_SETOWN, getpid());

flags = fcntl(fd, F_GETFL);
fcntl(fd, F_SETFL, flags|FASYNC); /* when low memory, receive SIGIO */
</usage example>
--
Zan Lynx <[email protected]>


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2008-01-26 01:56:24

by Bryan Henderson

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

>> Incidentally, some context for the AIX approach to the OOM problem: a
>> process may exclude itself from OOM vulnerability altogether. It
places
>> itself in "early allocation" mode, which means at the time it creates
>> virtual memory, it reserves enough backing store for the worst case.
The
>> memory manager does not send such a process the SIGDANGER signal or
>> terminate it when it runs out of paging space. Before c. 2000, this
was
>> the only mode. Now the default is late allocation mode, which is
similar
>> to Linux.
>
>This is an interesting approach. It feels like some programs might be
>interested in choosing this mode instead of risking OOM.

It's the way virtual memory always worked when it was first invented. The
system not only reserved space to back every page of virtual memory; it
assigned the particular blocks for it. Late allocation was a later
innovation, and I believe its main goal was to make it possible to use the
cheaper disk drives for paging instead of drums. Late allocation gives
you better locality on disk, so the seeking doesn't eat you alive (drums
don't seek). Even then, I assume (but am not sure) that the system at
least reserved the space in an account somewhere so at pageout time there
was guaranteed to be a place to which to page out. Overcommitting page
space to save on disk space was a later idea.

I was surprised to see AIX do late allocation by default, because IBM's
traditional style is bulletproof systems. A system where a process can be
killed at unpredictable times because of resource demands of unrelated
processes doesn't really fit that style.

It's really a fairly unusual application that benefits from late
allocation: one that creates a lot more virtual memory than it ever
touches. For example, a sparse array. Or am I missing something?

--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems

2008-01-26 11:56:24

by KOSAKI Motohiro

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

> The commentary on the mem_notify threads claimed that the signal is
> easily provided by setting up the file handle for SIGIO.

BTW:
Of cource, you can receive any signal instead SIGIO by use fcntl(F_SETSIG) :-)

2008-01-26 12:32:34

by KOSAKI Motohiro

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

> > And from a performance point of view letting applications voluntarily
> > free some memory is better even than starting to swap.
>
> Absolutely.

the mem_notify patch can realize "just before starting swapping" notification :)

to be honest, I don't know fs guys requirement.
if lacking feature of fs guys needed, I implement it with presure if
you tell me it.

2008-01-26 13:22:56

by Theodore Ts'o

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Fri, Jan 25, 2008 at 05:55:51PM -0800, Bryan Henderson wrote:
> I was surprised to see AIX do late allocation by default, because IBM's
> traditional style is bulletproof systems. A system where a process can be
> killed at unpredictable times because of resource demands of unrelated
> processes doesn't really fit that style.
>
> It's really a fairly unusual application that benefits from late
> allocation: one that creates a lot more virtual memory than it ever
> touches. For example, a sparse array. Or am I missing something?

I guess it depends on how far you try to do "bulletproof". OSF/1 used
to use "bulletproof" as its default --- and I had to turn it off on
tsx-11.mit.edu (the first North American ftp server for Linux :-),
because the difference was something like 50 ftp daemons versus over
500 on the same server. It reserved VM space for the text segement of
every single process, since at least in theory, it's possible for
every single text page to get modified using ptrace if (for example) a
debugger were to set a break point on every single page of every
single text segement of every single ftp daemon.

You can also see potential problems for Java programs. Suppose you
had some gigantic Java Application (say, Lotus Notes, or Websphere
Application Server) which is taking up many, many, MANY gigabytes of
VM space. Now suppose the Java application needs to fork and exec
some trivial helper program. For that tiny instant, between the fork
and exec, the VM requirements in "bulletproof" mode would double,
since while 99.9999% of the time programs will immediately discard the
VM upon the exec, there is always the possibility that the child
process will touch every single data page, forcing a copy on write,
and never do the exec.

There are of course different levels of "bulletproof" between the
extremes of "totally bulletproof" and "late binding" from an
algorithmic standpoint. For example, you could ignore the needed
pages caused by ptrace(); more challenging would be to how to handle
the fork/exec semantics, although there could be kludges such as
strongly encouraging applications to use an old-fashed BSD-style
vfork() to guarantee that the child couldn't double VM requirements
between the vfork() and exec(). I certainly can't say for sure what
the AIX designers had in mind, and why they didn't choose one of the
more intermediate design choices.

However, it is fair to say that "100% bulletproof" can require
reserving far more VM resources than you might first expect. Even a
company which is highly incented to sell large amounts of hardware,
such as Digital, might not have wanted their OS to be only able to
support an embarassingly small number of simultaneous ftpd
connections. I know this for sure because the OSF/1 documentation,
when discussing their VM tuning knobs, specifically talked about the
scenario that I ran into with tsx-11.mit.edu.

Regards,

- Ted

2008-01-28 19:57:20

by Theodore Ts'o

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Mon, Jan 28, 2008 at 07:30:05PM +0000, Pavel Machek wrote:
>
> As user pages are always in highmem, this should be easy to decide:
> only send SIGDANGER when highmem is full. (Yes, there are
> inodes/dentries/file descriptors in lowmem, but I doubt apps will
> respond to SIGDANGER by closing files).

Good point; for a system with at least (say) 2GB of memory, that
definitely makes sense. For a system with less than 768 megs of
memory (how quaint, but it wasn't that long ago this was a lot of
memory :-), there wouldn't *be* any memory in highmem at all....

- Ted

2008-01-29 07:18:12

by David Lang

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

On Mon, 28 Jan 2008, Theodore Tso wrote:

> On Mon, Jan 28, 2008 at 07:30:05PM +0000, Pavel Machek wrote:
>>
>> As user pages are always in highmem, this should be easy to decide:
>> only send SIGDANGER when highmem is full. (Yes, there are
>> inodes/dentries/file descriptors in lowmem, but I doubt apps will
>> respond to SIGDANGER by closing files).
>
> Good point; for a system with at least (say) 2GB of memory, that
> definitely makes sense. For a system with less than 768 megs of
> memory (how quaint, but it wasn't that long ago this was a lot of
> memory :-), there wouldn't *be* any memory in highmem at all....

not to mention machines with 1G of ram (900M lowmem, 128M highmem)

David Lang

2008-02-03 13:51:14

by KOSAKI Motohiro

[permalink] [raw]
Subject: Re: [RFC] Parallelize IO for e2fsck

Hi Pavel

> > > As user pages are always in highmem, this should be easy to decide:
> > > only send SIGDANGER when highmem is full. (Yes, there are
> > > inodes/dentries/file descriptors in lowmem, but I doubt apps will
> > > respond to SIGDANGER by closing files).
> >
> > Good point; for a system with at least (say) 2GB of memory, that
> > definitely makes sense. For a system with less than 768 megs of
> > memory (how quaint, but it wasn't that long ago this was a lot of
> > memory :-), there wouldn't *be* any memory in highmem at all....
>
> Ok, so it is 'send SIGDANGER when all zones are low', because user
> allocations can go from all zones (unless you have something really
> exotic, I'm not sure if that is true on huge NUMA machines & similar).

thank you good point out.

to be honest, the zone awareness of current mem_notify is premature.
I think we need enhancement rss statistics to per zone rss.
but not implemented yet ;-)

and, unfortunately I have no highmem machine.
the mem_notify is not so tested on highmem machine.

if you help to test, I am very happy!
Thanks.