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

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

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. BTW, It's kind of an
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

> Is the test completely skipping looking
>    at the packet otherwise?
> 
> 2. As long as we are doing this, see
> 	Alignment vs. Networking
> 	========================
> in Documentation/unaligned-memory-access.txt
> 
> 
> > From the micro-benchmarks results, we can expect +10% compared to
> > indirect descriptors, and + 5% compared to using 2 descs in the
> > virtqueue.
> > Also, it should have the same benefits as indirect descriptors for 0%
> > pkt loss (as we can fill 2x more packets in the virtqueue).
> > 
> > What do you think?
> > 
> > Thanks,
> > Maxime