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

[Yuanhan Liu]

On Tue, Oct 11, 2016 at 02:57:49PM +0800, Yuanhan Liu wrote:
> > > > > > There was an example: the vhost enqueue optmization patchset from
> > > > > > Zhihong [0] uses memset, and it introduces more than 15% drop (IIRC)

Though it doesn't matter now, but I have verified it yesterday (with and
wihtout memset), the drop could be up to 30+%.

This is to let you know that it could behaviour badly if memset is not
inlined.

> > > > > > 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.

I have another concern though: It's a trick could let gcc do the inline,
I am not quite sure whether that's ture with other compilers (i.e. clang,
icc, or even, older gcc).

For this case, I think I still prefer some trick like
    *(struct ..*) = {0, }

Or even, we may could introduce rte_memset(). IIRC, that has been
proposed somehow before?

	--yliu