Re: [Qemu-devel] [PATCH v3] migration: always initial ram_counters for a new migration

[Wei Yang <richardw.yang@linux.intel.com>]

On Mon, Aug 05, 2019 at 09:16:24AM +0800, Ivan Ren wrote:
>On Mon, Aug 5, 2019 at 8:34 AM Wei Yang <richardw.yang@linux.intel.com> wrote:
>>
>> On Fri, Aug 02, 2019 at 06:18:41PM +0800, Ivan Ren wrote:
>> >From: Ivan Ren <ivanren@tencent.com>
>> >
>> >This patch fix a multifd migration bug in migration speed calculation, this
>> >problem can be reproduced as follows:
>> >1. start a vm and give a heavy memory write stress to prevent the vm be
>> >   successfully migrated to destination
>> >2. begin a migration with multifd
>> >3. migrate for a long time [actually, this can be measured by transferred bytes]
>> >4. migrate cancel
>> >5. begin a new migration with multifd, the migration will directly run into
>> >   migration_completion phase
>> >
>> >Reason as follows:
>> >
>> >Migration update bandwidth and s->threshold_size in function
>> >migration_update_counters after BUFFER_DELAY time:
>> >
>> >    current_bytes = migration_total_bytes(s);
>> >    transferred = current_bytes - s->iteration_initial_bytes;
>> >    time_spent = current_time - s->iteration_start_time;
>> >    bandwidth = (double)transferred / time_spent;
>> >    s->threshold_size = bandwidth * s->parameters.downtime_limit;
>> >
>> >In multifd migration, migration_total_bytes function return
>> >qemu_ftell(s->to_dst_file) + ram_counters.multifd_bytes.
>> >s->iteration_initial_bytes will be initialized to 0 at every new migration,
>> >but ram_counters is a global variable, and history migration data will be
>> >accumulated. So if the ram_counters.multifd_bytes is big enough, it may lead
>> >pending_size >= s->threshold_size become false in migration_iteration_run
>> >after the first migration_update_counters.
>> >
>> >Signed-off-by: Ivan Ren <ivanren@tencent.com>
>> >Reviewed-by: Juan Quintela <quintela@redhat.com>
>> >Suggested-by: Wei Yang <richardw.yang@linux.intel.com>
>> >---
>> >v2->v3:
>> >- fix the bug of update_iteration_initial_status function prototype
>> >
>>
>> Code looks good. Have you verified on this version?
>
>Yes
>
>> BTW, you didn't address the multifd count in this patch, right?
>
>Yes.
>Currently multifd page count has no harm, so I think it's better to
>optimize it in a new patch to make things clearer.

Fine.

Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>

>
>Thanks.

-- 
Wei Yang
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