Re: [PATCH 0/5] Add vhost-blk support

[Stefan Hajnoczi]

On Tue, Jul 17, 2012 at 12:42 PM, Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
> On Tue, Jul 17, 2012 at 12:26 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>> On Tue, Jul 17, 2012 at 12:11:15PM +0100, Stefan Hajnoczi wrote:
>>> On Tue, Jul 17, 2012 at 10:21 AM, Asias He <asias@xxxxxxxxxx> wrote:
>>> > On 07/17/2012 04:52 PM, Paolo Bonzini wrote:
>>> >>
>>> >> Il 17/07/2012 10:29, Asias He ha scritto:
>>> >>>
>>> >>> So, vhost-blk at least saves ~6 syscalls for us in each request.
>>> >>
>>> >>
>>> >> Are they really 6?  If I/O is coalesced by a factor of 3, for example
>>> >> (i.e. each exit processes 3 requests), it's really 2 syscalls per request.
>>> >
>>> >
>>> > Well. I am counting the number of syscalls in one notify and response
>>> > process. Sure the IO can be coalesced.
>>>
>>> Linux AIO also supports batching in io_submit() and io_getevents().
>>> Depending on the request pattern in the vring when you process it, you
>>> should be able to do better than 1 set of syscalls per host I/O
>>> request.
>>>
>>> Are you taking advantage of that at the moment in your userspace benchmark?
>>>
>>> Stefan
>>
>> Injecting an interrupt directly from kernel bypasses two context switches.
>> Yes some worloads can coalesce interrupts efficiently but others can't.
>> It is not really hard to speculate more.
>>
>> Personally I don't understand where all this speculation leads us.
>> Are you guys disputing the measurements posted? If not would not
>> it be better if discussion focused on the amount of extra code versus
>> measured gain?
>
> 5-15% is nice.  But what causes the performance advantage?

To be clear, I suggest posting profiling results and explaining the
improvement over kvmtool.

Stefan
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