Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature

[Yuanhan Liu]

On Mon, Oct 10, 2016 at 07:39:59AM +0300, Michael S. Tsirkin wrote:
> > > > > > 1. why is this memset expensive?
> > > > >
> > > > > I don't have the exact answer, but just some rough thoughts:
> > > > >
> > > > > It's an external clib function: there is a call stack and the
> > > > > IP register will bounch back and forth.
> > > >
> > > > for memset 0?  gcc 5.3.1 on fedora happily inlines it.
> > > 
> > > Good to know!
> > > 
> > > > > overkill to use that for resetting 14 bytes structure.
> > > > >
> > > > > Some trick like
> > > > >     *(struct virtio_net_hdr *)hdr = {0, };
> > > > >
> > > > > Or even
> > > > >     hdr->xxx = 0;
> > > > >     hdr->yyy = 0;
> > > > >
> > > > > should behaviour better.
> > > > >
> > > > > There was an example: the vhost enqueue optmization patchset from
> > > > > Zhihong [0] uses memset, and it introduces more than 15% drop (IIRC)
> > > > > on my Ivybridge server: it has no such issue on his server though.
> > > > >
> > > > > [0]: http://dpdk.org/ml/archives/dev/2016-August/045272.html
> > > > >
> > > > >       --yliu
> > > >
> > > > I'd say that's weird. what's your config? any chance you
> > > > are using an old compiler?
> > > 
> > > Not really, it's gcc 5.3.1. Maybe Zhihong could explain more. IIRC,
> > > he said the memset is not well optimized for Ivybridge server.
> > 
> > The dst is remote in that case. It's fine on Haswell but has complication
> > in Ivy Bridge which (wasn't supposed to but) causes serious frontend issue.
> > 
> > I don't think gcc inlined it there. I'm using fc24 gcc 6.1.1.
> 
> 
> So try something like this then:

Yes, I saw memset is inlined when this diff is applied.

So, mind to send a formal patch? You might want to try build at least:
it doesn't build.

 
> Generally pointer chasing in vq->hw->vtnet_hdr_size can't be good
> for performance. Move fields used on data path into vq
> and use from there to avoid indirections?

Good suggestion!

        --yliu