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 ?
With regards,
Daniel
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