Re: [Qemu-devel] [PATCHv2 2/2] migration: do not overwrite zero pages

[Peter Lieven]

Am 10.06.2013 um 18:10 schrieb mdroth <address@hidden>:

> On Mon, Jun 10, 2013 at 12:14:20PM +0200, Peter Lieven wrote:
>> on incoming migration do not memset pages to zero if they already read as 
>> zero.
>> this will allocate a new zero page and consume memory unnecessarily. even
>> if we madvise a MADV_DONTNEED later this will only deallocate the memory
>> asynchronously.
>> 
>> Signed-off-by: Peter Lieven <address@hidden>
>> ---
>> arch_init.c |   14 ++++++++------
>> 1 file changed, 8 insertions(+), 6 deletions(-)
>> 
>> diff --git a/arch_init.c b/arch_init.c
>> index 08fccf6..cf4e1d5 100644
>> --- a/arch_init.c
>> +++ b/arch_init.c
>> @@ -832,14 +832,16 @@ static int ram_load(QEMUFile *f, void *opaque, int 
>> version_id)
>>             }
>> 
>>             ch = qemu_get_byte(f);
>> -            memset(host, ch, TARGET_PAGE_SIZE);
>> +            if (ch != 0 || !is_zero_page(host)) {
>> +                memset(host, ch, TARGET_PAGE_SIZE);
>> #ifndef _WIN32
>> -            if (ch == 0 &&
>> -                (!kvm_enabled() || kvm_has_sync_mmu()) &&
>> -                getpagesize() <= TARGET_PAGE_SIZE) {
>> -                qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
>> -            }
>> +                if (ch == 0 &&
>> +                    (!kvm_enabled() || kvm_has_sync_mmu()) &&
>> +                    getpagesize() <= TARGET_PAGE_SIZE) {
>> +                    qemu_madvise(host, TARGET_PAGE_SIZE, 
>> QEMU_MADV_DONTNEED);
>> +                }
> 
> Reviewed-by: Michael Roth <address@hidden>
> 
> Also CC'ing qemu-stable, but from what I gather this just mitigates the
> performance impact of 1/2 and isn't strictly necessary to fix migration?
> i.e. we can still do outgoing migration to 1.5.0 guests?

correct. the regression was introduced by the patch that was reverted in 1/2.
This patch is just a nice to have to avoid unnecessary allocation of zero pages.

Peter