Re: [Qemu-devel] [PATCH 4/4] migration: use dirty_rate_high_cnt more aggressively

[Felipe Franciosi]

+ Matthew Rosato, original reviewer of 070afca25

> On 25 May 2017, at 02:03, Peter Xu <address@hidden> wrote:
> 
> On Wed, May 24, 2017 at 05:10:03PM +0100, Felipe Franciosi wrote:
>> The commit message from 070afca25 suggests that dirty_rate_high_cnt
>> should be used more aggressively to start throttling after two
>> iterations instead of four. The code, however, only changes the auto
>> convergence behaviour to throttle after three iterations. This makes the
>> behaviour more aggressive by kicking off throttling after two iterations
>> as originally intended.
> 
> For this one, I don't think fixing the code to match the commit
> message is that important. Instead, for me this patch looks more like
> something "changed iteration loops from 3 to 2".

I agree. If we decide a v2 is needed I can amend the commit message accordingly.

> So the point is, what
> would be the best practical number for it. And when we change an
> existing value, we should have some reason, since it'll change
> behavior of existing user (though I'm not sure whether this one will
> affect much).

We've done a few tests with this internally using various workloads (DBs, 
synthetic mem stress, &c.). In summary, and along the lines with how Qemu 
implements auto convergence today, I would say this counter should be removed.

Consider that when live migrating a large-ish VM (100GB+ RAM), the network will 
be under stress for (at least) several minutes. At the same time, the sole 
purpose of this counter is to attempt convergence without having to throttle 
the guest. That is, it defers throttling in the hope that either the guest 
calms down or that the network gets less congested.

For real workloads, both cases are unlikely to happen. Throttling will 
eventually be needed otherwise the migration will not converge.

> I believe with higher dirty_rate_high_cnt, we have more smooth
> throttling, but it'll be slower in responding; While if lower or even
> remove it, we'll get very fast throttling response speed but I guess
> it may be more possible to report a false positive?

With a higher dirty_rate_high_cnt, migration will simply take longer (if not 
converging). For real workloads, it doesn't change how much throttling is 
required. For spiky or varied workloads, it gives a chance for migration to 
converge without throttling, at the cost of massive network stress and a longer 
overall migration time (which is bad user experience IMO).

> IMHO here 3 is
> okay since after all we are solving the problem of unconverged
> migration, so as long as we can converge, I think it'll be fine.

Based on 070afca25's commit message, Jason seemed to think that four was too 
much and meant to change it to two. As explained above, I'd be in favour of 
removing this counter altogether, or at least make it configurable (perhaps a 
#define would be enough an improvement for now). This patch is intended as a 
compromise by effectively using two.

Great feedback! Thanks again.
Felipe


> 
> Thanks,
> 
>> 
>> Signed-off-by: Felipe Franciosi <address@hidden>
>> ---
>> migration/ram.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/migration/ram.c b/migration/ram.c
>> index 1a3d9e6..26e03a5 100644
>> --- a/migration/ram.c
>> +++ b/migration/ram.c
>> @@ -708,7 +708,7 @@ static void migration_bitmap_sync(RAMState *rs)
>> 
>>             if ((rs->num_dirty_pages_period * TARGET_PAGE_SIZE >
>>                    (bytes_xfer_now - rs->bytes_xfer_prev) / 2) &&
>> -                (rs->dirty_rate_high_cnt++ >= 2)) {
>> +                (++rs->dirty_rate_high_cnt >= 2)) {
>>                     trace_migration_throttle();
>>                     rs->dirty_rate_high_cnt = 0;
>>                     mig_throttle_guest_down();
>> -- 
>> 1.9.5
>> 
> 
> -- 
> Peter Xu