Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg

From	Stefano Stabellini <sstabellini@kernel.org>
Newsgroups	linux.kernel
Subject	Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg
Date	2017-08-01 00:30 +0200
Message-ID	<u9rmi-3wp-37@gated-at.bofh.it> (permalink)
References	(2 earlier) <u7fyX-8aW-59@gated-at.bofh.it> <u7C2u-5CF-17@gated-at.bofh.it> <u7Cc9-5FZ-3@gated-at.bofh.it> <u7Exj-7hl-1@gated-at.bofh.it> <u7SqB-7mN-21@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Thu, 27 Jul 2017, Boris Ostrovsky wrote:
> On 07/26/2017 08:08 PM, Stefano Stabellini wrote:
> > On Wed, 26 Jul 2017, Boris Ostrovsky wrote:
> >>>> +			count++;
> >>>> +		else
> >>>> +			wait_event_interruptible(map->active.inflight_conn_req,
> >>>> +						 pvcalls_front_read_todo(map));
> >>>> +	}
> >>> Should we be using PVCALLS_FRONT_MAX_SPIN here? In sendmsg it is
> >>> counting non-sleeping iterations but here we are sleeping so
> >>> PVCALLS_FRONT_MAX_SPIN (5000) may take a while.
> >>>
> >>> In fact, what shouldn't this waiting be a function of MSG_DONTWAIT
> >> err, which it already is. But the question still stands (except for
> >> MSG_DONTWAIT).
> > The code (admittedly unintuitive) is busy-looping (non-sleeping) for
> > 5000 iterations *before* attempting to sleep. So in that regard, recvmsg
> > and sendmsg use PVCALLS_FRONT_MAX_SPIN in the same way: only for
> > non-sleeping iterations.
> >
> 
> OK.
> 
> Why not go directly into wait_event_interruptible()? I see you write in
> the commit message
> 
> If not enough data is available on the ring, rather than returning
> immediately or sleep-waiting, spin for up to 5000 cycles. This small
> optimization turns out to improve performance and latency significantly.
> 
> 
> Is this because of scheduling latency? I think this should be mentioned not just in the commit message but also as a comment in the code.

It tries to mitigate scheduling latencies on both ends (dom0 and domU)
when the ring buffer is the bottleneck (high bandwidth connections). But
to be honest with you, it's mostly beneficial in the sendmsg case,
because for recvmsg we also introduce a busy-wait in regular
circumstances, when no data is actually available. I confirmed this
statement with a quick iperf test. I'll remove the spin from recvmsg and
keep it in sendmsg.


> 
> (I also think it's not "not enough data" but rather "no data"?)

you are right

Thread

[PATCH v2 00/13] introduce the Xen PV Calls frontend Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
  [PATCH v2 13/13] xen: introduce a Kconfig option to enable the pvcalls frontend Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
  [PATCH v2 10/13] xen/pvcalls: implement poll command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
    Re: [PATCH v2 10/13] xen/pvcalls: implement poll command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-27 01:30 +0200
      Re: [PATCH v2 10/13] xen/pvcalls: implement poll command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 02:30 +0200
  [PATCH v2 06/13] xen/pvcalls: implement listen command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
  [PATCH v2 07/13] xen/pvcalls: implement accept command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
    Re: [PATCH v2 07/13] xen/pvcalls: implement accept command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 20:00 +0200
      Re: [PATCH v2 07/13] xen/pvcalls: implement accept command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 01:30 +0200
  [PATCH v2 01/13] xen/pvcalls: introduce the pvcalls xenbus frontend Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
    [PATCH v2 12/13] xen/pvcalls: implement frontend disconnect Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
    [PATCH v2 05/13] xen/pvcalls: implement bind command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
      Re: [PATCH v2 05/13] xen/pvcalls: implement bind command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 17:00 +0200
        Re: [PATCH v2 05/13] xen/pvcalls: implement bind command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 02:00 +0200
          Re: [PATCH v2 05/13] xen/pvcalls: implement bind command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-27 16:50 +0200
            Re: [PATCH v2 05/13] xen/pvcalls: implement bind command Stefano Stabellini <sstabellini@kernel.org> - 2017-08-01 00:20 +0200
    [PATCH v2 04/13] xen/pvcalls: implement connect command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
      Re: [PATCH v2 04/13] xen/pvcalls: implement connect command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 17:00 +0200
        Re: [PATCH v2 04/13] xen/pvcalls: implement connect command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 01:30 +0200
    [PATCH v2 09/13] xen/pvcalls: implement recvmsg Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
      Re: [PATCH v2 09/13] xen/pvcalls: implement recvmsg Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 23:30 +0200
        Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 23:40 +0200
          Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 02:10 +0200
            Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-27 17:00 +0200
              Re: [Xen-devel] [PATCH v2 09/13] xen/pvcalls: implement recvmsg Stefano Stabellini <sstabellini@kernel.org> - 2017-08-01 00:30 +0200
    [PATCH v2 11/13] xen/pvcalls: implement release command Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
      Re: [PATCH v2 11/13] xen/pvcalls: implement release command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-27 20:40 +0200
        Re: [PATCH v2 11/13] xen/pvcalls: implement release command Stefano Stabellini <sstabellini@kernel.org> - 2017-08-01 00:40 +0200
          Re: [PATCH v2 11/13] xen/pvcalls: implement release command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-08-01 17:30 +0200
            Re: [PATCH v2 11/13] xen/pvcalls: implement release command Juergen Gross <jgross@suse.com> - 2017-08-01 17:40 +0200
              Re: [PATCH v2 11/13] xen/pvcalls: implement release command Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-08-01 18:10 +0200
    [PATCH v2 02/13] xen/pvcalls: connect to the backend Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
      Re: [PATCH v2 02/13] xen/pvcalls: connect to the backend Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 15:40 +0200
        Re: [PATCH v2 02/13] xen/pvcalls: connect to the backend Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 02:30 +0200
          Re: [PATCH v2 02/13] xen/pvcalls: connect to the backend Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-27 17:10 +0200
            Re: [PATCH v2 02/13] xen/pvcalls: connect to the backend Stefano Stabellini <sstabellini@kernel.org> - 2017-08-01 00:00 +0200
    [PATCH v2 03/13] xen/pvcalls: implement socket command and handle events Stefano Stabellini <sstabellini@kernel.org> - 2017-07-25 23:30 +0200
      Re: [PATCH v2 03/13] xen/pvcalls: implement socket command and handle  events Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-07-26 16:30 +0200
        Re: [PATCH v2 03/13] xen/pvcalls: implement socket command and handle  events Stefano Stabellini <sstabellini@kernel.org> - 2017-07-27 01:20 +0200

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