* Peter Xu (peterx@redhat.com) wrote:
> Based on:
> [PATCH] tests: migration-test: Allow test to run without uffd
> https://lore.kernel.org/qemu-devel/20220707184600.24164-1-peterx@redhat.com/
>
> This is v8 of postcopy preempt series. It can also be found here:
> https://github.com/xzpeter/qemu/tree/postcopy-preempt
>
> RFC: https://lore.kernel.org/qemu-devel/20220119080929.39485-1-peterx@redhat.com
> V1: https://lore.kernel.org/qemu-devel/20220216062809.57179-1-peterx@redhat.com
> V2: https://lore.kernel.org/qemu-devel/20220301083925.33483-1-peterx@redhat.com
> V3: https://lore.kernel.org/qemu-devel/20220330213908.26608-1-peterx@redhat.com
> V4: https://lore.kernel.org/qemu-devel/20220331150857.74406-1-peterx@redhat.com
> V5: https://lore.kernel.org/qemu-devel/20220425233847.10393-1-peterx@redhat.com
> V6: https://lore.kernel.org/qemu-devel/20220517195730.32312-1-peterx@redhat.com
> V7: https://lore.kernel.org/qemu-devel/20220524221151.18225-1-peterx@redhat.com
> V8: https://lore.kernel.org/qemu-devel/20220622204920.79061-1-peterx@redhat.com
>
> v9:
> - Rebase upon latest master (plus the test fix above on "tests:
> migration-test: Allow test to run without uffd")
> - Added missing R-bs in v7
Queued, took some minor rework in the tests
> Abstract
> ========
>
> This series added a new migration capability called "postcopy-preempt". It can
> be enabled when postcopy is enabled, and it'll simply (but greatly) speed up
> postcopy page requests handling process.
>
> Below are some initial postcopy page request latency measurements after the
> new series applied.
>
> For each page size, I measured page request latency for three cases:
>
> (a) Vanilla: the old postcopy
> (b) Preempt no-break-huge: preempt enabled, x-postcopy-preempt-break-huge=off
> (c) Preempt full: preempt enabled, x-postcopy-preempt-break-huge=on
> (this is the default option when preempt enabled)
>
> Here x-postcopy-preempt-break-huge parameter is just added in v2 so as to
> conditionally disable the behavior to break sending a precopy huge page for
> debugging purpose. So when it's off, postcopy will not preempt precopy
> sending a huge page, but still postcopy will use its own channel.
>
> I tested it separately to give a rough idea on which part of the change
> helped how much of it. The overall benefit should be the comparison
> between case (a) and (c).
>
> |-----------+---------+-----------------------+--------------|
> | Page size | Vanilla | Preempt no-break-huge | Preempt full |
> |-----------+---------+-----------------------+--------------|
> | 4K | 10.68 | N/A [*] | 0.57 |
> | 2M | 10.58 | 5.49 | 5.02 |
> | 1G | 2046.65 | 933.185 | 649.445 |
> |-----------+---------+-----------------------+--------------|
> [*]: This case is N/A because 4K page does not contain huge page at all
>
> [1] https://github.com/xzpeter/small-stuffs/blob/master/tools/huge_vm/uffd-latency.bpf
>
> TODO List
> =========
>
> Avoid precopy write() blocks postcopy
> -------------------------------------
>
> I didn't prove this, but I always think the write() syscalls being blocked
> for precopy pages can affect postcopy services. If we can solve this
> problem then my wild guess is we can further reduce the average page
> latency.
>
> Two solutions at least in mind: (1) we could have made the write side of
> the migration channel NON_BLOCK too, or (2) multi-threads on send side,
> just like multifd, but we may use lock to protect which page to send too
> (e.g., the core idea is we should _never_ rely anything on the main thread,
> multifd has that dependency on queuing pages only on main thread).
>
> That can definitely be done and thought about later.
>
> Multi-channel for preemption threads
> ------------------------------------
>
> Currently the postcopy preempt feature use only one extra channel and one
> extra thread on dest (no new thread on src QEMU). It should be mostly good
> enough for major use cases, but when the postcopy queue is long enough
> (e.g. hundreds of vCPUs faulted on different pages) logically we could
> still observe more delays in average. Whether growing threads/channels can
> solve it is debatable, but sounds worthwhile a try. That's yet another
> thing we can think about after this patchset lands.
>
> Logically the design provides space for that - the receiving postcopy
> preempt thread can understand all ram-layer migration protocol, and for
> multi channel and multi threads we could simply grow that into multile
> threads handling the same protocol (with multiple PostcopyTmpPage). The
> source needs more thoughts on synchronizations, though, but it shouldn't
> affect the whole protocol layer, so should be easy to keep compatible.
>
> Please review, thanks.
>
> Peter Xu (14):
> migration: Add postcopy-preempt capability
> migration: Postcopy preemption preparation on channel creation
> migration: Postcopy preemption enablement
> migration: Postcopy recover with preempt enabled
> migration: Create the postcopy preempt channel asynchronously
> migration: Add property x-postcopy-preempt-break-huge
> migration: Add helpers to detect TLS capability
> migration: Export tls-[creds|hostname|authz] params to cmdline too
> migration: Enable TLS for preempt channel
> migration: Respect postcopy request order in preemption mode
> tests: Move MigrateCommon upper
> tests: Add postcopy tls migration test
> tests: Add postcopy tls recovery migration test
> tests: Add postcopy preempt tests
>
> migration/channel.c | 9 +-
> migration/migration.c | 134 ++++++++++++--
> migration/migration.h | 44 ++++-
> migration/multifd.c | 4 +-
> migration/postcopy-ram.c | 186 +++++++++++++++++++-
> migration/postcopy-ram.h | 11 ++
> migration/qemu-file.c | 27 +++
> migration/qemu-file.h | 1 +
> migration/ram.c | 326 +++++++++++++++++++++++++++++++++--
> migration/ram.h | 4 +-
> migration/savevm.c | 46 +++--
> migration/socket.c | 22 ++-
> migration/socket.h | 1 +
> migration/tls.c | 9 +
> migration/tls.h | 4 +
> migration/trace-events | 15 +-
> qapi/migration.json | 7 +-
> tests/qtest/migration-test.c | 286 +++++++++++++++++++++---------
> 18 files changed, 990 insertions(+), 146 deletions(-)
>
> --
> 2.32.0
>
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK