2008-07-30 19:21:11

by [email protected]

[permalink] [raw]
Subject: Re: high latency NFS

You might get more responses from the linux-nfs list (cc'd).

--b.

On Thu, Jul 24, 2008 at 01:11:31PM -0400, Michael Shuey wrote:
> I'm currently toying with Linux's NFS, to see just how fast it can go in a
> high-latency environment. Right now, I'm simulating a 100ms delay between
> client and server with netem (just 100ms on the outbound packets from the
> client, rather than 50ms each way). Oddly enough, I'm running into
> performance problems. :-)
>
> According to iozone, my server can sustain about 90/85 MB/s (reads/writes)
> without any latency added. After a pile of tweaks, and injecting 100ms of
> netem latency, I'm getting 6/40 MB/s (reads/writes). I'd really like to
> know why writes are now so much faster than reads, and what sort of things
> might boost the read throughput. Any suggestions?
> 1
> The read throughput seems to be proportional to the latency - adding only
> 10ms of delay gives 61 MB/s reads, in limited testing (need to look at it
> further). While that's to be expected, to some extent, I'm hoping there's
> some form of readahead that can help me out here (assume big sequential
> reads).
>
> iozone is reading/writing a file twice the size of memory on the client with
> a 32k block size. I've tried raising this as high as 16 MB, but I still
> see around 6 MB/sec reads.
>
> I'm using a 2.6.9 derivative (yes, I'm a RHEL4 fan). Testing with a stock
> 2.6, client and server, is the next order of business.
>
> NFS mount is tcp, version 3. rsize/wsize are 32k. Both client and server
> have had tcp_rmem, tcp_wmem, wmem_max, rmem_max, wmem_default, and
> rmem_default tuned - tuning values are 12500000 for defaults (and minimum
> window sizes), 25000000 for the maximums. Inefficient, yes, but I'm not
> concerned with memory efficiency at the moment.
>
> Both client and server kernels have been modified to provide
> larger-than-normal RPC slot tables. I allow a max of 1024, but I've found
> that actually enabling more than 490 entries in /proc causes mount to
> complain it can't allocate memory and die. That was somewhat suprising,
> given I had 122 GB of free memory at the time...
>
> I've also applied a couple patches to allow the NFS readahead to be a
> tunable number of RPC slots. Currently, I set this to 489 on client and
> server (so it's one less than the max number of RPC slots). Bandwidth
> delay product math says 380ish slots should be enough to keep a gigabit
> line full, so I suspect something else is preventing me from seeing the
> readahead I expect.
>
> FYI, client and server are connected via gigabit ethernet. There's a couple
> routers in the way, but they talk at 10gigE and can route wire speed.
> Traffic is IPv4, path MTU size is 9000 bytes.
>
> Is there anything I'm missing?
>
> --
> Mike Shuey
> Purdue University/ITaP
> --
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2008-07-30 21:56:07

by Shehjar Tikoo

[permalink] [raw]
Subject: Re: high latency NFS

J. Bruce Fields wrote:
> You might get more responses from the linux-nfs list (cc'd).
>
> --b.
>
> On Thu, Jul 24, 2008 at 01:11:31PM -0400, Michael Shuey wrote:
>> I'm currently toying with Linux's NFS, to see just how fast it
>> can go in a high-latency environment. Right now, I'm simulating
>> a 100ms delay between client and server with netem (just 100ms
>> on the outbound packets from the client, rather than 50ms each
>> way). Oddly enough, I'm running into performance problems. :-)
>>
>> According to iozone, my server can sustain about 90/85 MB/s
>> (reads/writes) without any latency added. After a pile of
>> tweaks, and injecting 100ms of netem latency, I'm getting 6/40
>> MB/s (reads/writes). I'd really like to know why writes are now
>> so much faster than reads, and what sort of things might boost
>> the read throughput. Any suggestions?

Is the server sync or async mounted? I've seen such performance
inversion between read and write when the mount mode is async.

What is the number of nfsd threads at the server?

Which file system are you using at the server?

>> 1 The read throughput seems to be proportional to the latency -
>> adding only 10ms of delay gives 61 MB/s reads, in limited testing
>> (need to look at it further). While that's to be expected, to
>> some extent, I'm hoping there's some form of readahead that can
>> help me out here (assume big sequential reads).
>>
>> iozone is reading/writing a file twice the size of memory on the
>> client with a 32k block size. I've tried raising this as high
>> as 16 MB, but I still see around 6 MB/sec reads.

In iozone, are you running the read and write test during the same run
of iozone? Iozone runs read tests, after writes so that the file for
the read test exists on the server. You should try running write and
read tests in separate runs to prevent client side caching issues from
influencing raw server read(and read-ahead) performance. You can use
the -w option in iozone to prevent iozone from calling unlink on the
file after the write test has finished, so you can use the same file
in a separate read test run.


>>
>> I'm using a 2.6.9 derivative (yes, I'm a RHEL4 fan). Testing
>> with a stock 2.6, client and server, is the next order of
>> business.

You can try building the kernel with oprofile support and use it to
measure where the client CPU is spending its time. It is possible that
client-side locking or other algorithm issues are resulting in such
low read throughput. Note, when you start oprofile profiling, use a
CPU_CYCLES count of 5000. I've observed more accurate results with
this sample size for NFS performance.

>>
>> NFS mount is tcp, version 3. rsize/wsize are 32k. Both client
>> and server have had tcp_rmem, tcp_wmem, wmem_max, rmem_max,
>> wmem_default, and rmem_default tuned - tuning values are 12500000
>> for defaults (and minimum window sizes), 25000000 for the
>> maximums. Inefficient, yes, but I'm not concerned with memory
>> efficiency at the moment.
>>
>> Both client and server kernels have been modified to provide
>> larger-than-normal RPC slot tables. I allow a max of 1024, but
>> I've found that actually enabling more than 490 entries in /proc
>> causes mount to complain it can't allocate memory and die. That
>> was somewhat suprising, given I had 122 GB of free memory at the
>> time...
>>
>> I've also applied a couple patches to allow the NFS readahead to
>> be a tunable number of RPC slots. Currently, I set this to 489
>> on client and server (so it's one less than the max number of
>> RPC slots). Bandwidth delay product math says 380ish slots
>> should be enough to keep a gigabit line full, so I suspect
>> something else is preventing me from seeing the readahead I
>> expect.
>>
>> FYI, client and server are connected via gigabit ethernet.
>> There's a couple routers in the way, but they talk at 10gigE and
>> can route wire speed. Traffic is IPv4, path MTU size is 9000
>> bytes.
>>

The following are not completely relevant here but just to get some
more info:
What is the raw TCP throughput that you get between the server and
client machine on this network?

You could run the tests with bare minimum number of network
elements between the server and the client to see whats the best
network performance for NFS you can extract from this server and
client machine.


>> Is there anything I'm missing?
>>
>> -- Mike Shuey Purdue University/ITaP

2008-07-31 02:35:49

by Michael Shuey

[permalink] [raw]
Subject: Re: high latency NFS

Thanks for all the tips I've received this evening. However, I figured out
the problem late last night. :-)

I was only using the default 8 nfsd threads on the server. When I raised
this to 256, the read bandwidth went from about 6 MB/sec to about 95
MB/sec, at 100ms of netem-induced latency. Not too shabby. I can get
about 993 Mbps on the gigE link between client and server, or 124 MB/sec
max, so this is about 76% of wire speed. Network connections pass through
three switches, at least one of which acts as a router, so I'm feeling
pretty good about things so far.

FYI, the server is using an ext3 file system, on top of a 10 GB /dev/ram0
ramdisk (exported async, mounted async). Oddly enough, /dev/ram0 seems a
bit slower than tmpfs and a loopback-mounted file - go figure.

To avoid confusing this with cache effects, I'm using iozone on an 8GB file
from a client with only 4GB of memory. Like I said, I'm mainly interested
in large file performance. :-)

--
Mike Shuey
Purdue University/ITaP


On Wednesday 30 July 2008, Shehjar Tikoo wrote:
> J. Bruce Fields wrote:
> > You might get more responses from the linux-nfs list (cc'd).
> >
> > --b.
> >
> > On Thu, Jul 24, 2008 at 01:11:31PM -0400, Michael Shuey wrote:
> >> I'm currently toying with Linux's NFS, to see just how fast it
> >> can go in a high-latency environment. Right now, I'm simulating
> >> a 100ms delay between client and server with netem (just 100ms
> >> on the outbound packets from the client, rather than 50ms each
> >> way). Oddly enough, I'm running into performance problems. :-)
> >>
> >> According to iozone, my server can sustain about 90/85 MB/s
> >> (reads/writes) without any latency added. After a pile of
> >> tweaks, and injecting 100ms of netem latency, I'm getting 6/40
> >> MB/s (reads/writes). I'd really like to know why writes are now
> >> so much faster than reads, and what sort of things might boost
> >> the read throughput. Any suggestions?
>
> Is the server sync or async mounted? I've seen such performance
> inversion between read and write when the mount mode is async.
>
> What is the number of nfsd threads at the server?
>
> Which file system are you using at the server?
>
> >> 1 The read throughput seems to be proportional to the latency -
> >> adding only 10ms of delay gives 61 MB/s reads, in limited testing
> >> (need to look at it further). While that's to be expected, to
> >> some extent, I'm hoping there's some form of readahead that can
> >> help me out here (assume big sequential reads).
> >>
> >> iozone is reading/writing a file twice the size of memory on the
> >> client with a 32k block size. I've tried raising this as high
> >> as 16 MB, but I still see around 6 MB/sec reads.
>
> In iozone, are you running the read and write test during the same run
> of iozone? Iozone runs read tests, after writes so that the file for
> the read test exists on the server. You should try running write and
> read tests in separate runs to prevent client side caching issues from
> influencing raw server read(and read-ahead) performance. You can use
> the -w option in iozone to prevent iozone from calling unlink on the
> file after the write test has finished, so you can use the same file
> in a separate read test run.
>
> >> I'm using a 2.6.9 derivative (yes, I'm a RHEL4 fan). Testing
> >> with a stock 2.6, client and server, is the next order of
> >> business.
>
> You can try building the kernel with oprofile support and use it to
> measure where the client CPU is spending its time. It is possible that
> client-side locking or other algorithm issues are resulting in such
> low read throughput. Note, when you start oprofile profiling, use a
> CPU_CYCLES count of 5000. I've observed more accurate results with
> this sample size for NFS performance.
>
> >> NFS mount is tcp, version 3. rsize/wsize are 32k. Both client
> >> and server have had tcp_rmem, tcp_wmem, wmem_max, rmem_max,
> >> wmem_default, and rmem_default tuned - tuning values are 12500000
> >> for defaults (and minimum window sizes), 25000000 for the
> >> maximums. Inefficient, yes, but I'm not concerned with memory
> >> efficiency at the moment.
> >>
> >> Both client and server kernels have been modified to provide
> >> larger-than-normal RPC slot tables. I allow a max of 1024, but
> >> I've found that actually enabling more than 490 entries in /proc
> >> causes mount to complain it can't allocate memory and die. That
> >> was somewhat suprising, given I had 122 GB of free memory at the
> >> time...
> >>
> >> I've also applied a couple patches to allow the NFS readahead to
> >> be a tunable number of RPC slots. Currently, I set this to 489
> >> on client and server (so it's one less than the max number of
> >> RPC slots). Bandwidth delay product math says 380ish slots
> >> should be enough to keep a gigabit line full, so I suspect
> >> something else is preventing me from seeing the readahead I
> >> expect.
> >>
> >> FYI, client and server are connected via gigabit ethernet.
> >> There's a couple routers in the way, but they talk at 10gigE and
> >> can route wire speed. Traffic is IPv4, path MTU size is 9000
> >> bytes.
>
> The following are not completely relevant here but just to get some
> more info:
> What is the raw TCP throughput that you get between the server and
> client machine on this network?
>
> You could run the tests with bare minimum number of network
> elements between the server and the client to see whats the best
> network performance for NFS you can extract from this server and
> client machine.
>
> >> Is there anything I'm missing?
> >>
> >> -- Mike Shuey Purdue University/ITaP

2008-08-04 08:11:47

by Greg Banks

[permalink] [raw]
Subject: Re: high latency NFS

Index: linux-2.6.16/net/sunrpc/svcsock.c
===================================================================
--- linux-2.6.16.orig/net/sunrpc/svcsock.c 2008-06-16 15:39:01.774672997 +1000
+++ linux-2.6.16/net/sunrpc/svcsock.c 2008-06-16 15:45:06.203421620 +1000
@@ -1157,13 +1159,13 @@ svc_tcp_recvfrom(struct svc_rqst *rqstp)
* particular pool, which provides an upper bound
* on the number of threads which will access the socket.
*
- * rcvbuf just needs to be able to hold a few requests.
- * Normally they will be removed from the queue
- * as soon a a complete request arrives.
+ * rcvbuf needs the same room as sndbuf, to allow
+ * workloads comprising mostly WRITE calls to flow
+ * at a reasonable fraction of line speed.
*/
svc_sock_setbufsize(svsk->sk_sock,
(serv->sv_nrthreads+3) * serv->sv_bufsz,
- 3 * serv->sv_bufsz);
+ (serv->sv_nrthreads+3) * serv->sv_bufsz);

svc_sock_clear_data_ready(svsk);


Attachments:
knfsd-tcp-receive-buffer-scaling (970.00 B)