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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 10, 2016 at 04:16:19AM +0000, Wang, Zhihong wrote:
> 
> 
> > -----Original Message-----
> > From: Yuanhan Liu [mailto:yuanhan.liu@linux.intel.com]
> > Sent: Monday, October 10, 2016 11:59 AM
> > To: Michael S. Tsirkin <mst@redhat.com>
> > Cc: Maxime Coquelin <maxime.coquelin@redhat.com>; Stephen Hemminger
> > <stephen@networkplumber.org>; dev@dpdk.org; qemu-
> > devel@nongnu.org; Wang, Zhihong <zhihong.wang@intel.com>
> > Subject: Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature
> > 
> > On Mon, Oct 10, 2016 at 06:46:44AM +0300, Michael S. Tsirkin wrote:
> > > On Mon, Oct 10, 2016 at 11:37:44AM +0800, Yuanhan Liu wrote:
> > > > On Thu, Sep 29, 2016 at 11:21:48PM +0300, Michael S. Tsirkin wrote:
> > > > > On Thu, Sep 29, 2016 at 10:05:22PM +0200, Maxime Coquelin wrote:
> > > > > >
> > > > > >
> > > > > > On 09/29/2016 07:57 PM, Michael S. Tsirkin wrote:
> > > > > Yes but two points.
> > > > >
> > > > > 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.

I just wrote some test code and compiled with gcc -O2,
it did get inlined.

It's probably this:
        uint16_t head_size = vq->hw->vtnet_hdr_size;
...
                memset(hdr, 0, head_size);
IOW head_size is not known to compiler.

Try sticking a bool there instead of vtnet_hdr_size, and
	 memset(hdr, 0, bigheader ? 10 : 12);


> > 
> > 	--yliu