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

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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.

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



-boris

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[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
    [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
    [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
    [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
    [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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