Re: [PATCH v5 00/21] migration: Postcopy Preemption

[manish.mishra]

On 26/04/22 5:08 am, Peter Xu wrote:
> This is v5 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
>
> v4->v5 changelog:
> - Fixed all checkpatch.pl warnings
> - Picked up leftover patches from Dan's tls test case series:
>    
> https://lore.kernel.org/qemu-devel/20220310171821.3724080-1-berrange@redhat.com/
> - Rebased to v7.0.0 tag, collected more R-bs from Dave/Dan
> - In migrate_fd_cleanup(), use g_clear_pointer() for s->hostname [Dan]
> - Mark postcopy-preempt capability for 7.1 not 7.0 [Dan]
> - Moved migrate_channel_requires_tls() into tls.[ch] [Dan]
> - Mention the bug-fixing side effect of patch "migration: Export
>    tls-[creds|hostname|authz] params to cmdline too" on tls_authz [Dan]
> - Use g_autoptr where proper [Dan]
> - Drop a few (probably over-cautious) asserts on local_err being set [Dan]
> - Quite a few renamings in the qtest in the last few test patches [Dan]
>
> Abstract
> ========
>
> This series contains two parts now:
>
>    (1) Leftover patches from Dan's tls unit tests v2, which is first half
>    (2) Leftover patches from my postcopy preempt v4, which is second half
>
> 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

Hi Peter, Just wanted understand what setup was used for these experiments like

number of vcpu, workload, network bandwidth so that i can make sense of these

numbers. Also i could not understand reason for so much difference between 
preempt

full and Preempt no-break-huge especially for 1G case, so if you please share 
little more

details on this.

> 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.
>
> Daniel P. Berrangé (9):
>    tests: fix encoding of IP addresses in x509 certs
>    tests: add more helper macros for creating TLS x509 certs
>    tests: add migration tests of TLS with PSK credentials
>    tests: add migration tests of TLS with x509 credentials
>    tests: convert XBZRLE migration test to use common helper
>    tests: convert multifd migration tests to use common helper
>    tests: add multifd migration tests of TLS with PSK credentials
>    tests: add multifd migration tests of TLS with x509 credentials
>    tests: ensure migration status isn't reported as failed
>
> Peter Xu (12):
>    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: Parameter 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
>    tests: Add postcopy tls migration test
>    tests: Add postcopy tls recovery migration test
>    tests: Add postcopy preempt tests
>
>   meson.build                          |   1 +
>   migration/channel.c                  |  10 +-
>   migration/migration.c                | 146 +++-
>   migration/migration.h                |  46 +-
>   migration/multifd.c                  |   7 +-
>   migration/postcopy-ram.c             | 186 ++++-
>   migration/postcopy-ram.h             |  11 +
>   migration/qemu-file.c                |  27 +
>   migration/qemu-file.h                |   1 +
>   migration/ram.c                      | 283 +++++++-
>   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                  |   8 +-
>   tests/qtest/meson.build              |  12 +-
>   tests/qtest/migration-helpers.c      |  13 +
>   tests/qtest/migration-helpers.h      |   1 +
>   tests/qtest/migration-test.c         | 997 ++++++++++++++++++++++++---
>   tests/unit/crypto-tls-psk-helpers.c  |  18 +-
>   tests/unit/crypto-tls-psk-helpers.h  |   1 +
>   tests/unit/crypto-tls-x509-helpers.c |  16 +-
>   tests/unit/crypto-tls-x509-helpers.h |  53 ++
>   tests/unit/test-crypto-tlssession.c  |  11 +-
>   27 files changed, 1779 insertions(+), 170 deletions(-)