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

[Juan Quintela]

Ivan Ren <address@hidden> wrote:
> From: Ivan Ren <address@hidden>
>
> 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 <address@hidden>

Reviewed-by: Juan Quintela <address@hidden>