Daniel P. Berrangé <berrange@xxxxxxxxxx> writes:
> On Thu, Oct 10, 2024 at 02:52:56PM -0300, Fabiano Rosas wrote:
>> Daniel P. Berrangé <berrange@xxxxxxxxxx> writes:
>>
>> > On Thu, Oct 10, 2024 at 12:06:51PM -0300, Fabiano Rosas wrote:
>> >> Daniel P. Berrangé <berrange@xxxxxxxxxx> writes:
>> >>
>> >> > On Thu, Aug 08, 2024 at 05:38:03PM -0600, Jim Fehlig via Devel wrote:
>> >> >> Introduce support for QEMU's new mapped-ram stream format [1].
>> >> >> mapped-ram is enabled by default if the underlying QEMU advertises
>> >> >> the mapped-ram migration capability. It can be disabled by changing
>> >> >> the 'save_image_version' setting in qemu.conf to version '2'.
>> >> >>
>> >> >> To use mapped-ram with QEMU:
>> >> >> - The 'mapped-ram' migration capability must be set to true
>> >> >> - The 'multifd' migration capability must be set to true and
>> >> >> the 'multifd-channels' migration parameter must set to 1
>> >> >> - QEMU must be provided an fdset containing the migration fd
>> >> >> - The 'migrate' qmp command is invoked with a URI referencing the
>> >> >> fdset and an offset where to start writing the data stream, e.g.
>> >> >>
>> >> >> {"execute":"migrate",
>> >> >> "arguments":{"detach":true,"resume":false,
>> >> >> "uri":"file:/dev/fdset/0,offset=0x11921"}}
>> >> >>
>> >> >> The mapped-ram stream, in conjunction with direct IO and multifd
>> >> >> support provided by subsequent patches, can significantly improve
>> >> >> the time required to save VM memory state. The following tables
>> >> >> compare mapped-ram with the existing, sequential save stream. In
>> >> >> all cases, the save and restore operations are to/from a block
>> >> >> device comprised of two NVMe disks in RAID0 configuration with
>> >> >> xfs (~8600MiB/s). The values in the 'save time' and 'restore time'
>> >> >> columns were scraped from the 'real' time reported by time(1). The
>> >> >> 'Size' and 'Blocks' columns were provided by the corresponding
>> >> >> outputs of stat(1).
>> >> >>
>> >> >> VM: 32G RAM, 1 vcpu, idle (shortly after boot)
>> >> >>
>> >> >> | save | restore |
>> >> >> | time | time | Size | Blocks
>> >> >> -----------------------+---------+---------+--------------+--------
>> >> >> legacy | 6.193s | 4.399s | 985744812 | 1925288
>> >> >> -----------------------+---------+---------+--------------+--------
>> >> >> mapped-ram | 5.109s | 1.176s | 34368554354 | 1774472
>> >> >
>> >> > I'm surprised by the restore time speed up, as I didn't think
>> >> > mapped-ram should make any perf difference without direct IO
>> >> > and multifd.
>> >> >
>> >> >> -----------------------+---------+---------+--------------+--------
>> >> >> legacy + direct IO | 5.725s | 4.512s | 985765251 | 1925328
>> >> >> -----------------------+---------+---------+--------------+--------
>> >> >> mapped-ram + direct IO | 4.627s | 1.490s | 34368554354 | 1774304
>> >> >
>> >> > Still somewhat surprised by the speed up on restore here too
>> >>
>> >> Hmm, I'm thinking this might be caused by zero page handling. The non
>> >> mapped-ram path has an extra buffer_is_zero() and memset() of the hva
>> >> page.
>> >>
>> >> Now, is it an issue that mapped-ram skips that memset? I assume guest
>> >> memory will always be clear at the start of migration. There won't be a
>> >> situation where the destination VM starts with memory already
>> >> dirty... *and* the save file is also different, otherwise it wouldn't
>> >> make any difference.
>> >
>> > Consider the snapshot use case. You're running the VM, so memory
>> > has arbitrary contents, now you restore to a saved snapshot. QEMU
>> > remains running this whole time and you can't assume initial
>> > memory is zeroed. Surely we need the memset ?
>>
>> Hmm, I probably have a big gap on my knowledge here, but savevm doesn't
>> hook into file migration, so there's no way to load a snapshot with
>> mapped-ram that I know of. Is this something that libvirt enables
>> somehow? There would be no -incoming on the cmdline.
>
> Opps, yes, i always forget savevm is off in its own little world.
>
> Upstream we've talking about making savevm be a facade around the
> 'migrate' command, but no one has ever made a PoC.
Yeah, that would be nice. Once I learn how the data ends up in the qcow2
image, maybe I can look into adding a new 'snapshot' migration mode to
QEMU.
>
> With regards,
> Daniel
