linux-kernel - Re: [PATCH 08/16] virtio_ring: virtqueue_add_outbuf / virtqueue_add

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Message-ID: <87lia9c9er.fsf@rustcorp.com.au>
Date:	Thu, 28 Feb 2013 15:38:44 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, Asias He <asias@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 08/16] virtio_ring: virtqueue_add_outbuf / virtqueue_add_inbuf.

"Michael S. Tsirkin" <mst@...hat.com> writes:
> On Fri, Feb 22, 2013 at 10:32:46AM +1030, Rusty Russell wrote:
>> "Michael S. Tsirkin" <mst@...hat.com> writes:
>> > On Tue, Feb 19, 2013 at 06:26:26PM +1030, Rusty Russell wrote:
>> >> These are specialized versions of virtqueue_add_buf(), which cover
>> >> over 50% of cases and are far clearer.
>> >> 
>> >> In particular, the scatterlists passed to these functions don't have
>> >> to be clean (ie. we ignore end markers).
>> >> 
>> >> FIXME: I'm not sure about the unclean sglist bit.  I had a more
>> >> ambitious one which conditionally ignored end markers in the iterator,
>> >> but it was ugly and I suspect this is just as fast.  Maybe we should
>> >> just fix all the drivers?
>> >> 
>> >> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
>> >
>> > Looking at code, it seems that most users really have a single sg, in
>> > low memory. So how about simply passing void * instead of sg? Whoever
>> > has multiple sgs can use the rich interface.
>> 
>> Good point, let's do that:
>> 1) Make virtqueue_add_outbuf()/inbuf() take a void * and len.
>> 2) Transfer users across to use that.
>> 3) Make everyone else use clean scatterlists with virtqueue_add_sgs[].
>> 4) Remove virtqueue_add_bufs().
>> 
>> > Long term we might optimize this unrolling some loops, I think
>> > I saw this giving a small performance gain for -net.
>> 
>> I *think* we could make virtqueue_add() an inline and implement an
>> virtqueue_add_outsg() wrapper and gcc will eliminate the loops for us.
>> But not sure it's worth the text bloat...
>> 
>> Cheers,
>> Rusty.
>
> inline is mostly useless nowdays...  We can make it a static function and
> let gcc decide.

I know I've said before that inline is the register keyword of the '90s.
But not at -O2 with i686-linux-gnu-gcc-4.7 (Ubuntu/Linaro
4.7.2-2ubuntu1) 4.7.2.

Without the inline keywords, it doesn't inline virtqueue_add, and thus
sg_next_chained and sg_next_add aren't inlined:

$ for i in `seq 50`; do /usr/bin/time --format=%U ./vringh_test --indirect --eventidx --parallel; done 2>&1 | stats --trim-outliers
Using CPUS 0 and 3
Guest: notified 39102-39145(39105), pinged 39060-39063(39063)
Host: notified 39060-39063(39063), pinged 19551-19581(19553)
3.050000-3.220000(3.136875)

With inline:

$ for i in `seq 50`; do /usr/bin/time --format=%U ./vringh_test --indirect --eventidx --parallel; done 2>&1 | stats --trim-outliers
Using CPUS 0 and 3
Guest: notified 39084-39148(39099), pinged 39062-39063(39062)
Host: notified 39062-39063(39062), pinged 19542-19574(19550)
2.940000-3.140000(3.014583)

Cheers,
Rusty.

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