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

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Date:	Fri, 01 Mar 2013 11:31:40 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	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 27/02/2013 12:21, Rusty Russell ha scritto:
>>>> >> 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 :(
>
> Did you check what actually happened that improved speed so much?

No, it was a random aside, I didn't dig into it.  Perhaps we should
revisit using -O3 on the entire kernel, or perhaps grab gcc 3.8 and see
how that performs.

But I'm implementing specialized virtqueue_add_outbuf() and
virtqueue_add_inbuf() which seem to get more improvement anyway (except
occasionally I get hangs in my tests, which I'm debugging now...)

Cheers,
Rusty.

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