Re: [Qemu-devel] [PATCHv3 7/9] migration: do not sent zero pages in bulk stage

[Peter Lieven <pl@kamp.de>]

Am 21.03.2013 um 20:26 schrieb Eric Blake <eblake@redhat.com>:

> On 03/21/2013 09:57 AM, Peter Lieven wrote:
>> during bulk stage of ram migration if a page is a
>> zero page do not send it at all.
>> the memory at the destination reads as zero anyway.
>> 
>> even if there is an madvise with QEMU_MADV_DONTNEED
>> at the target upon receipt of a zero page I have observed
>> that the target starts swapping if the memory is overcommitted.
>> it seems that the pages are dropped asynchronously.
>> 
>> Signed-off-by: Peter Lieven <pl@kamp.de>
>> ---
>> arch_init.c |   10 ++++++----
>> 1 file changed, 6 insertions(+), 4 deletions(-)
> 
>>             if (is_zero_page(p)) {
>>                 acct_info.dup_pages++;
>> -                bytes_sent = save_block_hdr(f, block, offset, cont,
>> -                                            RAM_SAVE_FLAG_COMPRESS);
>> -                qemu_put_byte(f, *p);
>> -                bytes_sent += 1;
>> +                if (!ram_bulk_stage) {
>> +                    bytes_sent = save_block_hdr(f, block, offset, cont,
>> +                                                RAM_SAVE_FLAG_COMPRESS);
>> +                    qemu_put_byte(f, 0);
>> +                }
>> +                bytes_sent++;
> 
> Logic is STILL wrong.  I pointed out in v2 that bytes_sent should not be
> incremented if you are not sending the page, so it needs to be inside
> the 'if (!ram_bulk_stage)'.

If its inside then bytes_sent will be -1 at the end if we skip a page. This would lead
to the raw page being sent. This way it is 0 what I think is correct.

> 
> Do we want to add a new migration statistic counter of how many zero
> pages we omitted sending during the bulk stage?

You mean sth like skipped zero pages?

I was also thinking of renaming dup_pages into zero_pages in the statistics,
but this could break someone relying on it so I left it as is.

Peter


> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>