Re: [PATCH 02/16] virtio_ring: virtqueue_add_sgs, to add multiple sgs.

[Rusty Russell]

"Michael S. Tsirkin" <mst@xxxxxxxxxx> writes:
> On Wed, Feb 27, 2013 at 05:58:37PM +1030, Rusty Russell wrote:
>> Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
>> >> 
>> >> 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

Oops.  This result (and the next) is bogus.  I was playing with -O3, and
accidentally left that in :(

The final result was 3.005208, ie. 3% slowdown.  Which almost makes it
worth duplicating the whole set of code :(

>> After removing the now-unnecessary chain-cleaning in add_buf:
>>         2.660000-2.830000(2.753542)user
>> 
>> Any questions?
>> Rusty.
>
> Sorry, so which patches are included in the last stage?
> Something I didn't make clear: I tested 2/16 (the patch I replied to).

I wanted to tidy them up, add commentry, and integrate your tool cleanup
patches first.  That's when I noticed my screwup.

I'll push them now, but I want to revisit to see if there's something
cleverer I can do...

Cheers,
Rusty.


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