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Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram
From: |
Juan Quintela |
Subject: |
Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining() |
Date: |
Wed, 04 Apr 2018 11:02:56 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) |
Peter Xu <address@hidden> wrote:
> On Sun, Apr 01, 2018 at 12:25:36AM +0530, Balamuruhan S wrote:
>> expected_downtime value is not accurate with dirty_pages_rate * page_size,
>> using ram_bytes_remaining would yeild it correct.
>>
>> Signed-off-by: Balamuruhan S <address@hidden>
>> ---
>> migration/migration.c | 3 +--
>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/migration/migration.c b/migration/migration.c
>> index 58bd382730..4e43dc4f92 100644
>> --- a/migration/migration.c
>> +++ b/migration/migration.c
>> @@ -2245,8 +2245,7 @@ static void migration_update_counters(MigrationState
>> *s,
>> * recalculate. 10000 is a small enough number for our purposes
>> */
>> if (ram_counters.dirty_pages_rate && transferred > 10000) {
>> - s->expected_downtime = ram_counters.dirty_pages_rate *
>> - qemu_target_page_size() / bandwidth;
>> + s->expected_downtime = ram_bytes_remaining() / bandwidth;
>
> This field was removed in e4ed1541ac ("savevm: New save live migration
> method: pending", 2012-12-20), in which remaing RAM was used.
Unrelated O:-)
> And it was added back in 90f8ae724a ("migration: calculate
> expected_downtime", 2013-02-22), in which dirty rate was used.
We didn't want to update the field if there haven't been enough activity.
> However I didn't find a clue on why we changed from using remaining
> RAM to using dirty rate... So I'll leave this question to Juan.
>
> Besides, I'm a bit confused on when we'll want such a value. AFAIU
> precopy is mostly used by setting up the target downtime before hand,
> so we should already know the downtime before hand. Then why we want
> to observe such a thing?
What that field means is how much time the system needs to send
everything that is pending.
I.e. if expected_downtime = 2seconds, it means that with current dirty
rate, if we set a downtime of 2 or bigger it is going to finish
migration.
It is a help for upper layers to decide that:
- they want a 1second downtime
- system calculates with current load that they need a 2second downtime
So they can decide:
- change the downtime to 2seconds (easy)
- change the apps running on the guest to dirty less memory (It dependes
on the guest, app, etc).
I don't know if anyone is using it at all.
Later, Juan.
- [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), Balamuruhan S, 2018/04/01
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), Balamuruhan S, 2018/04/04
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), Peter Xu, 2018/04/04
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), Balamuruhan S, 2018/04/04
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), Dr. David Alan Gilbert, 2018/04/09
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), David Gibson, 2018/04/09
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), Dr. David Alan Gilbert, 2018/04/10
- Re: [Qemu-devel] [PATCH] migration: calculate expected_downtime with ram_bytes_remaining(), David Gibson, 2018/04/10