Re: [Qemu-devel] [PATCH 1/8] migration: stop compressing page in migration thread

[Peter Xu]

On Fri, Mar 16, 2018 at 04:05:14PM +0800, Xiao Guangrong wrote:
> 
> Hi David,
> 
> Thanks for your review.
> 
> On 03/15/2018 06:25 PM, Dr. David Alan Gilbert wrote:
> 
> > >   migration/ram.c | 32 ++++++++++++++++----------------
> > 
> > Hi,
> >    Do you have some performance numbers to show this helps?  Were those
> > taken on a normal system or were they taken with one of the compression
> > accelerators (which I think the compression migration was designed for)?
> 
> Yes, i have tested it on my desktop, i7-4790 + 16G, by locally live migrate
> the VM which has 8 vCPUs + 6G memory and the max-bandwidth is limited to 350.
> 
> During the migration, a workload which has 8 threads repeatedly written total
> 6G memory in the VM. Before this patchset, its bandwidth is ~25 mbps, after
> applying, the bandwidth is ~50 mbps.

Hi, Guangrong,

Not really review comments, but I got some questions. :)

IIUC this patch will only change the behavior when last_sent_block
changed.  I see that the performance is doubled after the change,
which is really promising.  However I don't fully understand why it
brings such a big difference considering that IMHO current code is
sending dirty pages per-RAMBlock.  I mean, IMHO last_sent_block should
not change frequently?  Or am I wrong?

Another follow-up question would be: have you measured how long time
needed to compress a 4k page, and how many time to send it?  I think
"sending the page" is not really meaningful considering that we just
put a page into the buffer (which should be extremely fast since we
don't really flush it every time), however I would be curious on how
slow would compressing a page be.

Thanks,

> 
> BTW, Compression will use almost all valid bandwidth after all of our work
> which i will post it out part by part.
> 

-- 
Peter Xu