[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [PATCH v2 4/6] XBZRLE cache size should not be larger t
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [PATCH v2 4/6] XBZRLE cache size should not be larger than guest memory size |
Date: |
Thu, 30 Jan 2014 18:48:24 +0000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
* Orit Wasserman (address@hidden) wrote:
> On 01/30/2014 08:23 PM, Dr. David Alan Gilbert wrote:
> >* Orit Wasserman (address@hidden) wrote:
> >>Signed-off-by: Orit Wasserman <address@hidden>
> >>---
> >> migration.c | 7 +++++++
> >> 1 file changed, 7 insertions(+)
> >>
> >>diff --git a/migration.c b/migration.c
> >>index 46a7305..25add6f 100644
> >>--- a/migration.c
> >>+++ b/migration.c
> >>@@ -479,6 +479,13 @@ void qmp_migrate_set_cache_size(int64_t value, Error
> >>**errp)
> >> return;
> >> }
> >>
> >>+ /* Cache should not be larger than guest ram size */
> >
> >Why? (It's admittedly odd, but does it actually break something if it's
> >larger?)
> >
>
> Because how XBZRLE works, the idea is that for workload that changes the same
> pages
> frequently, we can reduce the amount of transferred data sent by sending only
> the diff.
> We also compress the diff itself.
>
> The cache is used to store the previous page so we can calculate the diff, so
> at most it will
> contain all the guest pages.
It's a hash based cache though isn't it - so there will be some contention for
a cache size==ram size case?
Also this does mean that you have to be a little careful to pick a sane XBZRLE
cache
size, since one that's too large will now fail; I can only see that being
a problem on a machine with a mix of huge and tiny VMs.
( I sent the reviewd-by tag separately.)
Dave
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
[Qemu-devel] [PATCH v2 3/6] migration:fix free XBZRLE decoded_buf wrong, Orit Wasserman, 2014/01/30
[Qemu-devel] [PATCH v2 5/6] Don't abort on out of memory when creating page cache, Orit Wasserman, 2014/01/30
[Qemu-devel] [PATCH v2 6/6] Don't abort on memory allocation error, Orit Wasserman, 2014/01/30