Re: [PATCH 10/20] qemu: Add support for mapped-ram on save

Daniel P. Berrangé <berrange(a)redhat.com&gt; 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.