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

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Date:	Wed, 3 Dec 2014 14:59:43 -0500
From:	Trond Myklebust <trond.myklebust@...marydata.com>
To:	Jeff Layton <jeff.layton@...marydata.com>
Cc:	Tejun Heo <tj@...nel.org>, NeilBrown <neilb@...e.de>,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Linux Kernel mailing list <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [RFC PATCH 00/14] nfsd/sunrpc: add support for a workqueue-based nfsd

On Wed, Dec 3, 2014 at 2:20 PM, Jeff Layton <jeff.layton@...marydata.com> wrote:
> On Wed, 3 Dec 2014 14:08:01 -0500
> Trond Myklebust <trond.myklebust@...marydata.com> 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?

> What I think I'm going to do on the next pass is have the job that
> enqueues the xprt instead try to find an svc_rqst. If it finds it,
> then it can go ahead and queue the work struct in it to do the receive
> and processing in a single go.
>
> If it can't find one, it'll queue the xprt's work to allocate one and
> then queue that to do all of the work as before. That will likely
> penalize the case where there isn't an available svc_rqst, but in the
> common case that there is one it should go quickly.


-- 
Trond Myklebust

Linux NFS client maintainer, PrimaryData

trond.myklebust@...marydata.com
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