Re: [Qemu-devel] Stalls on Live Migration of VMs with a lot of memory

[Peter Lieven <pl@dlh.net>]

On 04.01.2012 02:38, Shu Ming wrote:
> On 2012-1-4 2:04, Peter Lieven wrote:
>> Hi all,
>>
>> is there any known issue when migrating VMs with a lot of (e.g. 32GB) 
>> of memory.
>> It seems that there is some portion in the migration code which takes 
>> too much time when the number
>> of memory pages is large.
>>
>> Symptoms are: Irresponsive VNC connection, VM stalls and also 
>> irresponsive QEMU Monitor (via TCP).
>>
>> The problem seems to be worse on 10G connections between 2 Nodes (i 
>> already tried limiting the
>> bandwidth with the migrate_set_speed command) than on 1G connections.
> Is the migration  accomplished finally? How long will that be?  I did 
> a test on VM with 4G and it took me about two seconds.
maybe i should have been more precise. i use hugetblfs and memory pre 
allocation. so all 32G are allocated and most of them
are dup. one problem seems to be an issue that has already been observed 
in 2010 but not addressed.

the rate limiter only counts bytes transferred. if there a lot of dup 
pages we end up reading almost the whole ram in one
cycle. i already patched the source to exit the while loop in stage 2 if 
either the file rate limiter kicks in *or* there have
been more pages * PAGE_SIZE bytes read than the rate limit allows.

this has improved the situation a bit, but it i have a few other ideas 
where time could be saved.

peter

>
>
>>
>> The problem also seems to be worse in qemu-kvm-1.0 than in 
>> qemu-kvm-0.12.5.
>>
>> Any hints?
>>
>> Thanks,
>> Peter
>>
>>
>
>