Hi,
I got a problem report with fio where larger block size random reads
where markedly slower with buffered IO than with O_DIRECT, and the
initial thought was that perhaps this was some fio oddity. The reporter
eventually discovered that turning off the fadvise hint made it work
fine. So I took a look, and it seems we never do readahead for
FMODE_RANDOM even if the request size is larger than 1 page. That seems
like a bug, if an application is doing eg 16kb random reads, you want to
readahead the 12kb remaining data. On devices where smaller transfer
sizes are slower than larger ones, this can make a large difference.
This patch makes us readahead even for FMODE_RANDOM, iff we'll be
reading more pages in that single read. I ran a quick test here, and it
appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
being passed in or not).
Signed-off-by: Jens Axboe <[email protected]>
diff --git a/mm/readahead.c b/mm/readahead.c
index 337b20e..d4b201c 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -501,8 +501,11 @@ void page_cache_sync_readahead(struct address_space *mapping,
if (!ra->ra_pages)
return;
- /* be dumb */
- if (filp->f_mode & FMODE_RANDOM) {
+ /*
+ * Be dumb for files marked as randomly accessed, but do readahead
+ * inside the original request (req_size > 1).
+ */
+ if ((filp->f_mode & FMODE_RANDOM) && req_size == 1) {
force_page_cache_readahead(mapping, filp, offset, req_size);
return;
}
--
Jens Axboe
Hi Jens,
On Fri, Apr 02, 2010 at 02:31:51AM +0800, Jens Axboe wrote:
> Hi,
>
> I got a problem report with fio where larger block size random reads
> where markedly slower with buffered IO than with O_DIRECT, and the
> initial thought was that perhaps this was some fio oddity. The reporter
> eventually discovered that turning off the fadvise hint made it work
> fine. So I took a look, and it seems we never do readahead for
> FMODE_RANDOM even if the request size is larger than 1 page. That seems
> like a bug, if an application is doing eg 16kb random reads, you want to
> readahead the 12kb remaining data. On devices where smaller transfer
> sizes are slower than larger ones, this can make a large difference.
>
> This patch makes us readahead even for FMODE_RANDOM, iff we'll be
> reading more pages in that single read. I ran a quick test here, and it
> appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
> being passed in or not).
I guess the application is doing (at least partial) sequential reads,
while at the same time tell kernel with POSIX_FADV_RANDOM that it's
doing random reads.
If so, it's mainly the application's fault.
However the kernel can behave more smart and less "dumb" :)
It can inherit the current good behavior of "submit one single 16kb io
request for a 16kb random read() syscall", while still be able to
start _larger sized_ readahead if it's actually a sequential one.
> Signed-off-by: Jens Axboe <[email protected]>
>
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 337b20e..d4b201c 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -501,8 +501,11 @@ void page_cache_sync_readahead(struct address_space *mapping,
> if (!ra->ra_pages)
> return;
>
> - /* be dumb */
> - if (filp->f_mode & FMODE_RANDOM) {
> + /*
> + * Be dumb for files marked as randomly accessed, but do readahead
> + * inside the original request (req_size > 1).
> + */
> + if ((filp->f_mode & FMODE_RANDOM) && req_size == 1) {
> force_page_cache_readahead(mapping, filp, offset, req_size);
> return;
> }
The patch only fixes the (req_size != 1) case that exposed by your
application. A complete fix would be
@@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
if (!ra->ra_pages)
return;
- /* be dumb */
- if (filp->f_mode & FMODE_RANDOM) {
- force_page_cache_readahead(mapping, filp, offset, req_size);
- return;
- }
-
/* do read-ahead */
ondemand_readahead(mapping, ra, filp, false, offset, req_size);
}
And a more optimized patch would look like this. Note that only the
last chunk is necessary for bug fixing, and only this chunk can be
applied to vanilla 2.6.34-rc3.
If no problem, I'll submit a patch with only the last chunk for
2.6.34, and submit the remaining chunks for 2.6.35.
Thanks,
Fengguang
---
Subject: readahead: more smart readahead on POSIX_FADV_RANDOM
From: Wu Fengguang <[email protected]>
Date: Fri Apr 02 08:52:42 CST 2010
Some times user space applications will tell POSIX_FADV_RANDOM
while still doing some sequential reads.
The kernel can behave a bit smarter in this case, by letting the
readahead heuristics handle the POSIX_FADV_RANDOM case, but with
less aggressive assumption on sequential reads.
CC: Jens Axboe <[email protected]>
Signed-off-by: Wu Fengguang <[email protected]>
---
mm/readahead.c | 17 ++++++++---------
1 file changed, 8 insertions(+), 9 deletions(-)
--- linux.orig/mm/readahead.c 2010-04-02 08:43:53.000000000 +0800
+++ linux/mm/readahead.c 2010-04-02 09:00:51.000000000 +0800
@@ -664,6 +664,7 @@ ondemand_readahead(struct address_space
unsigned long max = max_sane_readahead(ra->ra_pages);
unsigned long tt; /* thrashing shreshold */
pgoff_t start;
+ bool random_hint = (filp && (filp->f_mode & FMODE_RANDOM));
/*
* start of file
@@ -671,7 +672,8 @@ ondemand_readahead(struct address_space
if (!offset) {
ra_set_pattern(ra, RA_PATTERN_INITIAL);
ra->start = offset;
- if ((ra->ra_flags & READAHEAD_LSEEK) && req_size <= max) {
+ if ((random_hint || (ra->ra_flags & READAHEAD_LSEEK)) &&
+ req_size <= max) {
ra->size = req_size;
ra->async_size = 0;
goto readit;
@@ -743,8 +745,11 @@ context_readahead:
} else
start = offset;
- tt = count_history_pages(mapping, ra, offset,
- READAHEAD_ASYNC_RATIO * max);
+ if (unlikely(random_hint))
+ tt = 0;
+ else
+ tt = count_history_pages(mapping, ra, offset,
+ READAHEAD_ASYNC_RATIO * max);
/*
* no history pages cached, could be
* - a random read
@@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
if (!ra->ra_pages)
return;
- /* be dumb */
- if (filp->f_mode & FMODE_RANDOM) {
- force_page_cache_readahead(mapping, filp, offset, req_size);
- return;
- }
-
/* do read-ahead */
ondemand_readahead(mapping, ra, filp, false, offset, req_size);
}
On Fri, Apr 02 2010, Wu Fengguang wrote:
> Hi Jens,
>
> On Fri, Apr 02, 2010 at 02:31:51AM +0800, Jens Axboe wrote:
> > Hi,
> >
> > I got a problem report with fio where larger block size random reads
> > where markedly slower with buffered IO than with O_DIRECT, and the
> > initial thought was that perhaps this was some fio oddity. The reporter
> > eventually discovered that turning off the fadvise hint made it work
> > fine. So I took a look, and it seems we never do readahead for
> > FMODE_RANDOM even if the request size is larger than 1 page. That seems
> > like a bug, if an application is doing eg 16kb random reads, you want to
> > readahead the 12kb remaining data. On devices where smaller transfer
> > sizes are slower than larger ones, this can make a large difference.
> >
> > This patch makes us readahead even for FMODE_RANDOM, iff we'll be
> > reading more pages in that single read. I ran a quick test here, and it
> > appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
> > being passed in or not).
>
> I guess the application is doing (at least partial) sequential reads,
> while at the same time tell kernel with POSIX_FADV_RANDOM that it's
> doing random reads.
>
> If so, it's mainly the application's fault.
The application is doing large random reads. It's purely random, so
the POSIX_FADV_RANDOM hint is correct. However, thinking about it, it
may be that we later hit a random "block" that has now been cached due
to this read-ahead. Let me try and rule that out completely and see if
there's still the difference. The original reporter observed 4kb reads
hitting the driver, where 128kb was expected.
> However the kernel can behave more smart and less "dumb" :)
> It can inherit the current good behavior of "submit one single 16kb io
> request for a 16kb random read() syscall", while still be able to
> start _larger sized_ readahead if it's actually a sequential one.
Yeah, that's an ancient issue and pretty sad.
> > diff --git a/mm/readahead.c b/mm/readahead.c
> > index 337b20e..d4b201c 100644
> > --- a/mm/readahead.c
> > +++ b/mm/readahead.c
> > @@ -501,8 +501,11 @@ void page_cache_sync_readahead(struct address_space *mapping,
> > if (!ra->ra_pages)
> > return;
> >
> > - /* be dumb */
> > - if (filp->f_mode & FMODE_RANDOM) {
> > + /*
> > + * Be dumb for files marked as randomly accessed, but do readahead
> > + * inside the original request (req_size > 1).
> > + */
> > + if ((filp->f_mode & FMODE_RANDOM) && req_size == 1) {
> > force_page_cache_readahead(mapping, filp, offset, req_size);
> > return;
> > }
>
> The patch only fixes the (req_size != 1) case that exposed by your
> application. A complete fix would be
>
> @@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
> if (!ra->ra_pages)
> return;
>
> - /* be dumb */
> - if (filp->f_mode & FMODE_RANDOM) {
> - force_page_cache_readahead(mapping, filp, offset, req_size);
> - return;
> - }
> -
Hmm, are we talking about the same thing? I want to hit read-ahead for
the remaining pages inside that random read, eg ensure that read-ahead
gets activated inside that window of the random request.
> /* do read-ahead */
> ondemand_readahead(mapping, ra, filp, false, offset, req_size);
> }
>
> And a more optimized patch would look like this. Note that only the
> last chunk is necessary for bug fixing, and only this chunk can be
> applied to vanilla 2.6.34-rc3.
>
> If no problem, I'll submit a patch with only the last chunk for
> 2.6.34, and submit the remaining chunks for 2.6.35.
>
> Thanks,
> Fengguang
> ---
> Subject: readahead: more smart readahead on POSIX_FADV_RANDOM
> From: Wu Fengguang <[email protected]>
> Date: Fri Apr 02 08:52:42 CST 2010
>
> Some times user space applications will tell POSIX_FADV_RANDOM
> while still doing some sequential reads.
>
> The kernel can behave a bit smarter in this case, by letting the
> readahead heuristics handle the POSIX_FADV_RANDOM case, but with
> less aggressive assumption on sequential reads.
I'll try and give this a spin. On the laptop, I cannot reproduce the
problem of smaller < reqsize ios, so hard to say just now.
>
> CC: Jens Axboe <[email protected]>
> Signed-off-by: Wu Fengguang <[email protected]>
> ---
> mm/readahead.c | 17 ++++++++---------
> 1 file changed, 8 insertions(+), 9 deletions(-)
>
> --- linux.orig/mm/readahead.c 2010-04-02 08:43:53.000000000 +0800
> +++ linux/mm/readahead.c 2010-04-02 09:00:51.000000000 +0800
> @@ -664,6 +664,7 @@ ondemand_readahead(struct address_space
> unsigned long max = max_sane_readahead(ra->ra_pages);
> unsigned long tt; /* thrashing shreshold */
> pgoff_t start;
> + bool random_hint = (filp && (filp->f_mode & FMODE_RANDOM));
>
> /*
> * start of file
> @@ -671,7 +672,8 @@ ondemand_readahead(struct address_space
> if (!offset) {
> ra_set_pattern(ra, RA_PATTERN_INITIAL);
> ra->start = offset;
> - if ((ra->ra_flags & READAHEAD_LSEEK) && req_size <= max) {
> + if ((random_hint || (ra->ra_flags & READAHEAD_LSEEK)) &&
> + req_size <= max) {
> ra->size = req_size;
> ra->async_size = 0;
> goto readit;
> @@ -743,8 +745,11 @@ context_readahead:
> } else
> start = offset;
>
> - tt = count_history_pages(mapping, ra, offset,
> - READAHEAD_ASYNC_RATIO * max);
> + if (unlikely(random_hint))
> + tt = 0;
> + else
> + tt = count_history_pages(mapping, ra, offset,
> + READAHEAD_ASYNC_RATIO * max);
> /*
> * no history pages cached, could be
> * - a random read
> @@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
> if (!ra->ra_pages)
> return;
>
> - /* be dumb */
> - if (filp->f_mode & FMODE_RANDOM) {
> - force_page_cache_readahead(mapping, filp, offset, req_size);
> - return;
> - }
> -
> /* do read-ahead */
> ondemand_readahead(mapping, ra, filp, false, offset, req_size);
> }
--
Jens Axboe
On Fri, Apr 02, 2010 at 02:38:30PM +0800, Jens Axboe wrote:
> On Fri, Apr 02 2010, Wu Fengguang wrote:
> > Hi Jens,
> >
> > On Fri, Apr 02, 2010 at 02:31:51AM +0800, Jens Axboe wrote:
> > > Hi,
> > >
> > > I got a problem report with fio where larger block size random reads
> > > where markedly slower with buffered IO than with O_DIRECT, and the
> > > initial thought was that perhaps this was some fio oddity. The reporter
> > > eventually discovered that turning off the fadvise hint made it work
> > > fine. So I took a look, and it seems we never do readahead for
> > > FMODE_RANDOM even if the request size is larger than 1 page. That seems
> > > like a bug, if an application is doing eg 16kb random reads, you want to
> > > readahead the 12kb remaining data. On devices where smaller transfer
> > > sizes are slower than larger ones, this can make a large difference.
> > >
> > > This patch makes us readahead even for FMODE_RANDOM, iff we'll be
> > > reading more pages in that single read. I ran a quick test here, and it
> > > appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
> > > being passed in or not).
> >
> > I guess the application is doing (at least partial) sequential reads,
> > while at the same time tell kernel with POSIX_FADV_RANDOM that it's
> > doing random reads.
> >
> > If so, it's mainly the application's fault.
>
> The application is doing large random reads. It's purely random, so
> the POSIX_FADV_RANDOM hint is correct. However, thinking about it, it
How large is it? For random reads > read_ahead_kb,
ondemand_readahead() will break it into read_ahead_kb sized IOs, while
force_page_cache_readahead() won't. That may impact IO performance.
> may be that we later hit a random "block" that has now been cached due
> to this read-ahead. Let me try and rule that out completely and see if
> there's still the difference. The original reporter observed 4kb reads
> hitting the driver, where 128kb was expected.
4kb reads hit the disk (on POSIX_FADV_RANDOM)? That sounds like
behavior in pre .34 kernels that is fixed by commit 0141450f66c:
readahead: introduce FMODE_RANDOM for POSIX_FADV_RANDOM
> > However the kernel can behave more smart and less "dumb" :)
> > It can inherit the current good behavior of "submit one single 16kb io
> > request for a 16kb random read() syscall", while still be able to
> > start _larger sized_ readahead if it's actually a sequential one.
>
> Yeah, that's an ancient issue and pretty sad.
>
> > > diff --git a/mm/readahead.c b/mm/readahead.c
> > > index 337b20e..d4b201c 100644
> > > --- a/mm/readahead.c
> > > +++ b/mm/readahead.c
> > > @@ -501,8 +501,11 @@ void page_cache_sync_readahead(struct address_space *mapping,
> > > if (!ra->ra_pages)
> > > return;
> > >
> > > - /* be dumb */
> > > - if (filp->f_mode & FMODE_RANDOM) {
> > > + /*
> > > + * Be dumb for files marked as randomly accessed, but do readahead
> > > + * inside the original request (req_size > 1).
> > > + */
> > > + if ((filp->f_mode & FMODE_RANDOM) && req_size == 1) {
> > > force_page_cache_readahead(mapping, filp, offset, req_size);
> > > return;
> > > }
> >
> > The patch only fixes the (req_size != 1) case that exposed by your
> > application. A complete fix would be
> >
> > @@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
> > if (!ra->ra_pages)
> > return;
> >
> > - /* be dumb */
> > - if (filp->f_mode & FMODE_RANDOM) {
> > - force_page_cache_readahead(mapping, filp, offset, req_size);
> > - return;
> > - }
> > -
>
> Hmm, are we talking about the same thing? I want to hit read-ahead for
> the remaining pages inside that random read, eg ensure that read-ahead
> gets activated inside that window of the random request.
I think Yes. When the above block is gone, ondemand_readahead() will
be invoked, and the readahead heuristic will find that it's an
oversize read (whose size is > 128k) and start 128kb readahead for it.
Thanks,
Fengguang
> > /* do read-ahead */
> > ondemand_readahead(mapping, ra, filp, false, offset, req_size);
> > }
> >
> > And a more optimized patch would look like this. Note that only the
> > last chunk is necessary for bug fixing, and only this chunk can be
> > applied to vanilla 2.6.34-rc3.
> >
> > If no problem, I'll submit a patch with only the last chunk for
> > 2.6.34, and submit the remaining chunks for 2.6.35.
> >
> > Thanks,
> > Fengguang
> > ---
> > Subject: readahead: more smart readahead on POSIX_FADV_RANDOM
> > From: Wu Fengguang <[email protected]>
> > Date: Fri Apr 02 08:52:42 CST 2010
> >
> > Some times user space applications will tell POSIX_FADV_RANDOM
> > while still doing some sequential reads.
> >
> > The kernel can behave a bit smarter in this case, by letting the
> > readahead heuristics handle the POSIX_FADV_RANDOM case, but with
> > less aggressive assumption on sequential reads.
>
> I'll try and give this a spin. On the laptop, I cannot reproduce the
> problem of smaller < reqsize ios, so hard to say just now.
>
> >
> > CC: Jens Axboe <[email protected]>
> > Signed-off-by: Wu Fengguang <[email protected]>
> > ---
> > mm/readahead.c | 17 ++++++++---------
> > 1 file changed, 8 insertions(+), 9 deletions(-)
> >
> > --- linux.orig/mm/readahead.c 2010-04-02 08:43:53.000000000 +0800
> > +++ linux/mm/readahead.c 2010-04-02 09:00:51.000000000 +0800
> > @@ -664,6 +664,7 @@ ondemand_readahead(struct address_space
> > unsigned long max = max_sane_readahead(ra->ra_pages);
> > unsigned long tt; /* thrashing shreshold */
> > pgoff_t start;
> > + bool random_hint = (filp && (filp->f_mode & FMODE_RANDOM));
> >
> > /*
> > * start of file
> > @@ -671,7 +672,8 @@ ondemand_readahead(struct address_space
> > if (!offset) {
> > ra_set_pattern(ra, RA_PATTERN_INITIAL);
> > ra->start = offset;
> > - if ((ra->ra_flags & READAHEAD_LSEEK) && req_size <= max) {
> > + if ((random_hint || (ra->ra_flags & READAHEAD_LSEEK)) &&
> > + req_size <= max) {
> > ra->size = req_size;
> > ra->async_size = 0;
> > goto readit;
> > @@ -743,8 +745,11 @@ context_readahead:
> > } else
> > start = offset;
> >
> > - tt = count_history_pages(mapping, ra, offset,
> > - READAHEAD_ASYNC_RATIO * max);
> > + if (unlikely(random_hint))
> > + tt = 0;
> > + else
> > + tt = count_history_pages(mapping, ra, offset,
> > + READAHEAD_ASYNC_RATIO * max);
> > /*
> > * no history pages cached, could be
> > * - a random read
> > @@ -820,12 +825,6 @@ void page_cache_sync_readahead(struct ad
> > if (!ra->ra_pages)
> > return;
> >
> > - /* be dumb */
> > - if (filp->f_mode & FMODE_RANDOM) {
> > - force_page_cache_readahead(mapping, filp, offset, req_size);
> > - return;
> > - }
> > -
> > /* do read-ahead */
> > ondemand_readahead(mapping, ra, filp, false, offset, req_size);
> > }
>
> --
> Jens Axboe
On Fri, Apr 02 2010, Wu Fengguang wrote:
> On Fri, Apr 02, 2010 at 02:38:30PM +0800, Jens Axboe wrote:
> > On Fri, Apr 02 2010, Wu Fengguang wrote:
> > > Hi Jens,
> > >
> > > On Fri, Apr 02, 2010 at 02:31:51AM +0800, Jens Axboe wrote:
> > > > Hi,
> > > >
> > > > I got a problem report with fio where larger block size random reads
> > > > where markedly slower with buffered IO than with O_DIRECT, and the
> > > > initial thought was that perhaps this was some fio oddity. The reporter
> > > > eventually discovered that turning off the fadvise hint made it work
> > > > fine. So I took a look, and it seems we never do readahead for
> > > > FMODE_RANDOM even if the request size is larger than 1 page. That seems
> > > > like a bug, if an application is doing eg 16kb random reads, you want to
> > > > readahead the 12kb remaining data. On devices where smaller transfer
> > > > sizes are slower than larger ones, this can make a large difference.
> > > >
> > > > This patch makes us readahead even for FMODE_RANDOM, iff we'll be
> > > > reading more pages in that single read. I ran a quick test here, and it
> > > > appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
> > > > being passed in or not).
> > >
> > > I guess the application is doing (at least partial) sequential reads,
> > > while at the same time tell kernel with POSIX_FADV_RANDOM that it's
> > > doing random reads.
> > >
> > > If so, it's mainly the application's fault.
> >
> > The application is doing large random reads. It's purely random, so
> > the POSIX_FADV_RANDOM hint is correct. However, thinking about it, it
>
> How large is it? For random reads > read_ahead_kb,
> ondemand_readahead() will break it into read_ahead_kb sized IOs, while
> force_page_cache_readahead() won't. That may impact IO performance.
The test case was 128kb random reads. So should still be within the
normal read_ahead_kb. I suspect the reporter would not have noticed if
the issue size was as large as read_ahead_kb even if the request size
was larger, the problem was that he ended up seeing 4kb ios only.
> > may be that we later hit a random "block" that has now been cached due
> > to this read-ahead. Let me try and rule that out completely and see if
> > there's still the difference. The original reporter observed 4kb reads
> > hitting the driver, where 128kb was expected.
>
> 4kb reads hit the disk (on POSIX_FADV_RANDOM)? That sounds like
> behavior in pre .34 kernels that is fixed by commit 0141450f66c:
>
> readahead: introduce FMODE_RANDOM for POSIX_FADV_RANDOM
Could explain why I'm not reproducing when doing a quick test on the
laptop. It is an older kernel. So it could be that I'm just imaging the
issue on the current kernel, I don't have hard data to back it up on
that version.
So disregard the patch for now, part-sequential behaviour on
POSIX_FADV_RANDOM isn't the issue here.
--
Jens Axboe
On Fri, Apr 02, 2010 at 02:59:17PM +0800, Jens Axboe wrote:
> On Fri, Apr 02 2010, Wu Fengguang wrote:
> > On Fri, Apr 02, 2010 at 02:38:30PM +0800, Jens Axboe wrote:
> > > On Fri, Apr 02 2010, Wu Fengguang wrote:
> > > > Hi Jens,
> > > >
> > > > On Fri, Apr 02, 2010 at 02:31:51AM +0800, Jens Axboe wrote:
> > > > > Hi,
> > > > >
> > > > > I got a problem report with fio where larger block size random reads
> > > > > where markedly slower with buffered IO than with O_DIRECT, and the
> > > > > initial thought was that perhaps this was some fio oddity. The reporter
> > > > > eventually discovered that turning off the fadvise hint made it work
> > > > > fine. So I took a look, and it seems we never do readahead for
> > > > > FMODE_RANDOM even if the request size is larger than 1 page. That seems
> > > > > like a bug, if an application is doing eg 16kb random reads, you want to
> > > > > readahead the 12kb remaining data. On devices where smaller transfer
> > > > > sizes are slower than larger ones, this can make a large difference.
> > > > >
> > > > > This patch makes us readahead even for FMODE_RANDOM, iff we'll be
> > > > > reading more pages in that single read. I ran a quick test here, and it
> > > > > appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
> > > > > being passed in or not).
> > > >
> > > > I guess the application is doing (at least partial) sequential reads,
> > > > while at the same time tell kernel with POSIX_FADV_RANDOM that it's
> > > > doing random reads.
> > > >
> > > > If so, it's mainly the application's fault.
> > >
> > > The application is doing large random reads. It's purely random, so
> > > the POSIX_FADV_RANDOM hint is correct. However, thinking about it, it
> >
> > How large is it? For random reads > read_ahead_kb,
> > ondemand_readahead() will break it into read_ahead_kb sized IOs, while
> > force_page_cache_readahead() won't. That may impact IO performance.
>
> The test case was 128kb random reads. So should still be within the
> normal read_ahead_kb. I suspect the reporter would not have noticed if
Yeah. 128kb random reads won't trigger readahead.
However each 129kb random read will trigger 2*128kb readahead IOs,
if we let ondemand_readahead() handle these random reads..
> the issue size was as large as read_ahead_kb even if the request size
> was larger, the problem was that he ended up seeing 4kb ios only.
>
> > > may be that we later hit a random "block" that has now been cached due
> > > to this read-ahead. Let me try and rule that out completely and see if
> > > there's still the difference. The original reporter observed 4kb reads
> > > hitting the driver, where 128kb was expected.
> >
> > 4kb reads hit the disk (on POSIX_FADV_RANDOM)? That sounds like
> > behavior in pre .34 kernels that is fixed by commit 0141450f66c:
> >
> > readahead: introduce FMODE_RANDOM for POSIX_FADV_RANDOM
>
> Could explain why I'm not reproducing when doing a quick test on the
> laptop. It is an older kernel. So it could be that I'm just imaging the
> issue on the current kernel, I don't have hard data to back it up on
> that version.
>
> So disregard the patch for now, part-sequential behaviour on
> POSIX_FADV_RANDOM isn't the issue here.
OK.
Thanks,
Fengguang