Re: [Qemu-devel] [PATCH 0/7] migration: Optimization the xbzrle and fix two corruption issues

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Gonglei (arei.gonglei@huawei.com) wrote:
> On 2014/2/28 17:19, Dr. David Alan Gilbert wrote:
> 
> > * Gonglei (Arei) (arei.gonglei@huawei.com) wrote:
> > 
> > Hi,
> > 
> >> a. Optimization the xbzrle remarkable decrease the cache misses.
> >>     The efficiency of compress increases more than fifty times.
> >>     Before the patch set, the cache almost totally miss when the 
> >>     number of cache item less than the dirty page number. Now the
> >>     hot pages in the cache will not be replaced by other pages. 
> > 
> > Nice, what do you use as your performance test case for xbzrle?
> 
> 
> The VM we used with 25G memory and 1Gbit nic. We run a test procedure
> in the vm, which as this:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #define PAGE_SIZE 4096
> void main(void)
> {
>     char *p,*p1;
>     long i,j,z;
> 
>     p = (char*)calloc(8*1024,1024*1024);
>     if( p == NULL ){
>         printf("fail to calloc \n");
>         exit(1);
>     }
>     for(;;){
>         p1 = p;
>         for( i = 0 ; i < 8*1024 ; i++ ){
> 
>             for( j = 0 ; j < 1024*1024 ; j+=PAGE_SIZE ){
>                *p1 = 0x55;
>                p1+=PAGE_SIZE;
>             }
>         }
>     }
> }
> 
> finally,
> the results of without enable xbzrle: 115MB/sec
> 
> using xbzrle without optimization (the size of cache 2G): 116MB/sec
> 
> using xbzrle with our optimization (the size of cache 2G): 150MB/sec

Hmm yes, it's not a very realistic test is it.
Having said that, I've not managed to find a reallistic test people
can agree on; I was hoping you had one!

You're listing the differences in in MB/sec - what about total
time to migrate?

However, the other question is why your optimisation works well
with that test; is it just the CPU overhead that it's reducing
because it's not bothering to copy lots of stuff into the cache?
If that's all the guest is running, I can't see that it would
actually XBZRLE much - maybe just OS pages.
What do the 'info migrate' stats look like with/without your
optimisation - I'm interested in how many xbzrle pages are sent?

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK