linux-kernel - Re: [PATCH 02/16] virtio_ring: virtqueue_add

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Date:	Wed, 27 Feb 2013 17:58:37 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 02/16] virtio_ring: virtqueue_add_sgs, to add multiple sgs.

Paolo Bonzini <pbonzini@...hat.com> writes:
> Il 24/02/2013 23:12, Michael S. Tsirkin ha scritto:
>> On Tue, Feb 19, 2013 at 06:26:20PM +1030, Rusty Russell wrote:
>>> virtio_scsi can really use this, to avoid the current hack of copying
>>> the whole sg array.  Some other things get slightly neater, too.
>>>
>>> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>> 
>> Hmm, this makes add_buf a bit slower. virtio_test results
>> (I'll send a patch to update the test shortly):
>> 
>> Before:
>> 0.09user 0.01system 0:00.12elapsed 91%CPU (0avgtext+0avgdata 480maxresident)k
>> 0inputs+0outputs (0major+145minor)pagefaults 0swaps
>> 
>> After:
>> 0.11user 0.01system 0:00.13elapsed 90%CPU (0avgtext+0avgdata 480maxresident)k
>> 0inputs+0outputs (0major+145minor)pagefaults 0swaps
>
> Not unexpected at all... :(
>
> Some of it can be recovered, but if it's 20% I doubt all of it.  So my
> patches were not premature optimization; you really can take just two
> among speed, flexibility, and having a nice API.

The error bars on this are far too large to say "20%".

Here are my numbers, using 50 runs of:
time tools/virtio/vringh_test --indirect --eventidx --parallel and
stats --trim-outliers:

Baseline (before add_sgs):
        2.840000-3.040000(2.927292)user

After add_sgs:
        2.970000-3.150000(3.053750)user

After simplifying add_buf a little:
        2.950000-3.210000(3.081458)user

After inlining virtqueue_add/vring_add_indirect:
        2.920000-3.150000(3.026875)user

After passing in iteration functions (chained vs unchained):
        2.760000-2.970000(2.883542)user

After removing the now-unnecessary chain-cleaning in add_buf:
        2.660000-2.830000(2.753542)user

Any questions?
Rusty.
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