Hi, netdev folks. The summary here is:
A patch added in the 2.6.30 development cycle caused a performance
regression in my NFS iozone testing. The patch in question is the
following:
commit 47a14ef1af48c696b214ac168f056ddc79793d0e
Author: Olga Kornievskaia <aglo@xxxxxxxxxxxxxx>
Date: Tue Oct 21 14:13:47 2008 -0400
svcrpc: take advantage of tcp autotuning
which is also quoted below. Using 8 nfsd threads, a single client doing
2GB of streaming read I/O goes from 107590 KB/s under 2.6.29 to 65558
KB/s under 2.6.30-rc4. I also see more run to run variation under
2.6.30-rc4 using the deadline I/O scheduler on the server. That
variation disappears (as does the performance regression) when reverting
the above commit.
Olga had the following to say about the regression:
>> On Wed, 13 May 2009 12:20:57 -0400 Olga Kornievskaia <aglo@xxxxxxxxxxxxxx> wrote:
>>
>>> I believe what you are seeing is how well TCP autotuning performs.
>>> What old NFS code was doing is disabling autotuning and instead using
>>> #nfsd thread to scale TCP recv window. You are providing an example of
>>> where setting TCP buffer sizes outperforms TCP autotuning. While this
>>> is a valid example, there is also an alternative example of where old
>>> NFS design hurts performance.
> We realize that decrease performance is a problem and understand that
> reverting the patch might be the appropriate course of action!
> But we are curious why this is happening. Jeff if it's not too much trouble
> could you generate tcpdumps for both cases. We are curious what are
> the max window sizes in both cases? Also could you give us your tcp and
> network sysctl values for the testing environment (both client and server
> values) that you can get with "sysctl -a | grep tcp" and also
> " | grep net.core".
The requested data can be found here:
http://people.redhat.com/jmoyer/iozone-regression.tar
> Poor performance using TCP autotuning can be demonstrated outside
> of NFS but using Iperf. It can be shown that iperf will work better if "-w"
> flag is used. When this flag is set, Iperf calls setsockopt() call which in
> the kernel turns off autotuning.
>
> As for fixing this it would be great if we could get some help from the
> TCP kernel folks?
And so I've CC'd netdev.
Jim Rees had the following to add:
> TCP autotuning can reduce performance by up to about 10% in some cases.
> Jeff found one of these cases. While the autotuning penalty never exceeds
> 10% as far as I know, I can provide examples of other cases where autotuning
> improves nfsd performance by more than a factor of 100.
>
> The right thing is to fix autotuning. If autotuning is considered too
> broken to use, it should be turned off everywhere, not just in nfsd, as it
> hurts/benefits all TCP clients, not just nfs.
> This topic has been discussed before on netdev:
> http://www.spinics.net/lists/netdev/msg68650.html
> http://www.spinics.net/lists/netdev/msg68155.html
I'd like to point out that the penalty is much greater than 10% in my
case.
Here are the data points:
Jeff Moyer <jmoyer@xxxxxxxxxx> writes:
>>> >> >> Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>>> >> >>
>>> >> >> > Hi,
>>> >> >> >
>>> >> >> > I've been working on CFQ improvements for interleaved I/Os between
>>> >> >> > processes, and noticed a regression in performance when using the
>>> >> >> > deadline I/O scheduler. The test uses a server configured with a cciss
>>> >> >> > array and 1Gb/s ethernet.
>>> >> >> >
>>> >> >> > The iozone command line was:
>>> >> >> > iozone -s 2000000 -r 64 -f /mnt/test/testfile -i 1 -w
>>> >> >> >
>>> >> >> > The numbers in the nfsd's row represent the number of nfsd "threads".
>>> >> >> > These numbers (in MB/s) represent the average of 5 runs.
>>> >> >> >
>>> >> >> > v2.6.29
>>> >> >> >
>>> >> >> > nfsd's | 1 | 2 | 4 | 8
>>> >> >> > --------+---------------+-------+------
>>> >> >> > deadline| 43207 | 67436 | 96289 | 107590
>>> >> >> >
>>> >> >> > 2.6.30-rc1
>>> >> >> >
>>> >> >> > nfsd's | 1 | 2 | 4 | 8
>>> >> >> > --------+---------------+-------+------
>>> >> >> > deadline| 43732 | 68059 | 76659 | 83231
>>> >> >> >
>>> >> >> > 2.6.30-rc3.block-for-linus
>>> >> >> >
>>> >> >> > nfsd's | 1 | 2 | 4 | 8
>>> >> >> > --------+---------------+-------+------
>>> >> >> > deadline| 46102 | 71151 | 83120 | 82330
>>> >> >> >
>>> >> >> >
>>> >> >> > Notice the drop for 4 and 8 threads. It may be worth noting that the
>>> >> >> > default number of NFSD threads is 8.
>
> Just following up with numbers:
>
> 2.6.30-rc4
>
> nfsd's | 8
> --------+------
> cfq | 51632 (49791 52436 52308 51488 52141)
> deadline| 65558 (41675 42559 74820 87518 81221)
>
> 2.6.30-rc4 reverting the sunrpc "fix"
>
> nfsd's | 8
> --------+------
> cfq | 82513 (81650 82762 83147 82935 82073)
> deadline| 107827 (109730 106077 107175 108524 107632)
>
> The numbers in parenthesis are the individual runs. Notice how
> 2.6.30-rc4 has some pretty wide variations for deadline.
>
>>> OK, I bisected this to the following commit. The mount is done using
>>> NFSv3, by the way.
>>>
>>> commit 47a14ef1af48c696b214ac168f056ddc79793d0e
>>> Author: Olga Kornievskaia <aglo@xxxxxxxxxxxxxx>
>>> Date: Tue Oct 21 14:13:47 2008 -0400
>>>
>>> svcrpc: take advantage of tcp autotuning
>>>
>>> Allow the NFSv4 server to make use of TCP autotuning behaviour, which
>>> was previously disabled by setting the sk_userlocks variable.
>>>
>>> Set the receive buffers to be big enough to receive the whole RPC
>>> request, and set this for the listening socket, not the accept socket.
>>>
>>> Remove the code that readjusts the receive/send buffer sizes for the
>>> accepted socket. Previously this code was used to influence the TCP
>>> window management behaviour, which is no longer needed when autotuning
>>> is enabled.
>>>
>>> This can improve IO bandwidth on networks with high bandwidth-delay
>>> products, where a large tcp window is required. It also simplifies
>>> performance tuning, since getting adequate tcp buffers previously
>>> required increasing the number of nfsd threads.
>>>
>>> Signed-off-by: Olga Kornievskaia <aglo@xxxxxxxxxxxxxx>
>>> Cc: Jim Rees <rees@xxxxxxxxx>
>>> Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxxxxxx>
>>>
>>> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
>>> index 5763e64..7a2a90f 100644
>>> --- a/net/sunrpc/svcsock.c
>>> +++ b/net/sunrpc/svcsock.c
>>> @@ -345,7 +345,6 @@ static void svc_sock_setbufsize(struct socket *sock, unsigned int snd,
>>> lock_sock(sock->sk);
>>> sock->sk->sk_sndbuf = snd * 2;
>>> sock->sk->sk_rcvbuf = rcv * 2;
>>> - sock->sk->sk_userlocks |= SOCK_SNDBUF_LOCK|SOCK_RCVBUF_LOCK;
>>> release_sock(sock->sk);
>>> #endif
>>> }
>>> @@ -797,23 +796,6 @@ static int svc_tcp_recvfrom(struct svc_rqst *rqstp)
>>> test_bit(XPT_CONN, &svsk->sk_xprt.xpt_flags),
>>> test_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags));
>>>
>>> - if (test_and_clear_bit(XPT_CHNGBUF, &svsk->sk_xprt.xpt_flags))
>>> - /* sndbuf needs to have room for one request
>>> - * per thread, otherwise we can stall even when the
>>> - * network isn't a bottleneck.
>>> - *
>>> - * We count all threads rather than threads in a
>>> - * 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.
>>> - */
>>> - svc_sock_setbufsize(svsk->sk_sock,
>>> - (serv->sv_nrthreads+3) * serv->sv_max_mesg,
>>> - 3 * serv->sv_max_mesg);
>>> -
>>> clear_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
>>>
>>> /* Receive data. If we haven't got the record length yet, get
>>> @@ -1061,15 +1043,6 @@ static void svc_tcp_init(struct svc_sock *svsk, struct svc_serv *serv)
>>>
>>> tcp_sk(sk)->nonagle |= TCP_NAGLE_OFF;
>>>
>>> - /* initialise setting must have enough space to
>>> - * receive and respond to one request.
>>> - * svc_tcp_recvfrom will re-adjust if necessary
>>> - */
>>> - svc_sock_setbufsize(svsk->sk_sock,
>>> - 3 * svsk->sk_xprt.xpt_server->sv_max_mesg,
>>> - 3 * svsk->sk_xprt.xpt_server->sv_max_mesg);
>>> -
>>> - set_bit(XPT_CHNGBUF, &svsk->sk_xprt.xpt_flags);
>>> set_bit(XPT_DATA, &svsk->sk_xprt.xpt_flags);
>>> if (sk->sk_state != TCP_ESTABLISHED)
>>> set_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags);
>>> @@ -1140,8 +1113,14 @@ static struct svc_sock *svc_setup_socket(struct svc_serv *serv,
>>> /* Initialize the socket */
>>> if (sock->type == SOCK_DGRAM)
>>> svc_udp_init(svsk, serv);
>>> - else
>>> + else {
>>> + /* initialise setting must have enough space to
>>> + * receive and respond to one request.
>>> + */
>>> + svc_sock_setbufsize(svsk->sk_sock, 4 * serv->sv_max_mesg,
>>> + 4 * serv->sv_max_mesg);
>>> svc_tcp_init(svsk, serv);
>>> + }
>>>
>>> /*
>>> * We start one listener per sv_serv. We want AF_INET
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