Re: [RFC PATCH 00/14] nfsd/sunrpc: add support for a workqueue-based nfsd

From: Trond Myklebust
Date: Wed Dec 03 2014 - 15:46:03 EST

On Wed, Dec 3, 2014 at 3:21 PM, Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx> wrote:
> On Wed, 3 Dec 2014 14:59:43 -0500
> Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote:
>> On Wed, Dec 3, 2014 at 2:20 PM, Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx> wrote:
>> > On Wed, 3 Dec 2014 14:08:01 -0500
>> > Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx> wrote:
>> >> Which workqueue are you using? Since the receive code is non-blocking,
>> >> I'd expect you might be able to use rpciod, for the initial socket
>> >> reads, but you wouldn't want to use that for the actual knfsd
>> >> processing.
>> >>
>> >
>> > I'm using the same (nfsd) workqueue for everything. The workqueue
>> > isn't really the bottleneck though, it's the work_struct.
>> >
>> > Basically, the problem is that the work_struct in the svc_xprt was
>> > remaining busy for far too long. So, even though the XPT_BUSY bit had
>> > cleared, the work wouldn't get picked up again until the previous
>> > workqueue job had returned.
>> >
>> > With the change I made today, I just added a new work_struct to
>> > svc_rqst and queue that to the same workqueue to do svc_process as soon
>> > as the receive is done. That means though that each RPC ends up waiting
>> > in the queue twice (once to do the receive and once to process the
>> > RPC), and I think that's probably the reason for the performance delta.
>> Why would the queuing latency still be significant now?
> That, I'm not clear on yet and that may not be why this is slower. But,
> I was seeing slightly faster performance with reads before I made
> today's changes. If changing how these jobs get queued doesn't help the
> performance, then I'll have to look elsewhere...

Do you have a good method for measuring that latency? If the queuing
latency turns out to depend on the execution latency for each job,
then perhaps running the message receives on a separate low latency
queue could help (hence the suggestion to use rpciod).

Trond Myklebust

Linux NFS client maintainer, PrimaryData

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