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Re: [Qemu-devel] migration: broken ram_save_pending
From: |
Alexey Kardashevskiy |
Subject: |
Re: [Qemu-devel] migration: broken ram_save_pending |
Date: |
Thu, 06 Feb 2014 14:10:35 +1100 |
User-agent: |
Mozilla/5.0 (X11; Linux i686 on x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0 |
On 02/06/2014 03:45 AM, Paolo Bonzini wrote:
> Il 05/02/2014 17:42, Dr. David Alan Gilbert ha scritto:
>> Because:
>> * the code is still running and keeps redirtying a small handful of
>> pages
>> * but because we've underestimated our available bandwidth we never stop
>> it and just throw those pages across immediately
>
> Ok, I thought Alexey was saying we are not redirtying that handful of pages.
Every iteration we read the dirty map from KVM and send all dirty pages
across the stream.
> And in turn, this is because the max downtime we have is too low
> (especially for the default 32 MB/sec default bandwidth; that's also pretty
> low).
My understanding nooow is that in order to finish migration QEMU waits for
the earliest 100ms (BUFFER_DELAY) of continuously low trafic but due to
those pages getting dirty every time we read the dirty map, we transfer
more in these 100ms than we are actually allowed (>32MB/s or 320KB/100ms).
So we transfer-transfer-transfer, detect than we transfer too much, do
delay() and if max_size (calculated from actual transfer and downtime) for
the next iteration is less (by luck) than those 96 pages (uncompressed) -
we finish.
Increasing speed or/and downtime will help but still - we would not need
that if migration did not expect all 96 pages to have to be sent but did
have some smart way to detect that many are empty (so - compressed).
Literally, move is_zero_range() from ram_save_block() to
migration_bitmap_sync() and store this bit in some new pages_zero_map, for
example. But does it make a lot of sense?
--
Alexey
- Re: [Qemu-devel] migration: broken ram_save_pending, (continued)
- Re: [Qemu-devel] migration: broken ram_save_pending, Alexey Kardashevskiy, 2014/02/04
- Re: [Qemu-devel] migration: broken ram_save_pending, Paolo Bonzini, 2014/02/04
- Re: [Qemu-devel] migration: broken ram_save_pending, Alexey Kardashevskiy, 2014/02/04
- Re: [Qemu-devel] migration: broken ram_save_pending, Paolo Bonzini, 2014/02/04
- Re: [Qemu-devel] migration: broken ram_save_pending, Alexey Kardashevskiy, 2014/02/04
- Re: [Qemu-devel] migration: broken ram_save_pending, Paolo Bonzini, 2014/02/05
- Re: [Qemu-devel] migration: broken ram_save_pending, Dr. David Alan Gilbert, 2014/02/05
- Re: [Qemu-devel] migration: broken ram_save_pending, Paolo Bonzini, 2014/02/05
- Re: [Qemu-devel] migration: broken ram_save_pending, Dr. David Alan Gilbert, 2014/02/05
- Re: [Qemu-devel] migration: broken ram_save_pending, Paolo Bonzini, 2014/02/05
- Re: [Qemu-devel] migration: broken ram_save_pending,
Alexey Kardashevskiy <=
- Re: [Qemu-devel] migration: broken ram_save_pending, Dr. David Alan Gilbert, 2014/02/06
- Re: [Qemu-devel] migration: broken ram_save_pending, Alexey Kardashevskiy, 2014/02/07
- Re: [Qemu-devel] migration: broken ram_save_pending, Dr. David Alan Gilbert, 2014/02/07
- Re: [Qemu-devel] migration: broken ram_save_pending, Paolo Bonzini, 2014/02/06
- Re: [Qemu-devel] migration: broken ram_save_pending, Alexey Kardashevskiy, 2014/02/07