Re: [PATCH v10 0/7] MSG_ZEROCOPY + multifd

[Dr. David Alan Gilbert]

* Leonardo Bras (leobras@redhat.com) wrote:
> This patch series intends to enable MSG_ZEROCOPY in QIOChannel, and make
> use of it for multifd migration performance improvement, by reducing cpu
> usage.
> 
> Patch #1 creates new callbacks for QIOChannel, allowing the implementation
> of zero copy writing.
> 
> Patch #2 implements io_writev flags and io_flush() on QIOChannelSocket,
> making use of MSG_ZEROCOPY on Linux.
> 
> Patch #3 adds a "zero_copy_send" migration property, only available with
> CONFIG_LINUX, and compiled-out in any other architectures.
> This migration property has to be enabled before multifd migration starts.
> 
> Patch #4 adds a helper function that allows to see if TLS is going to be used.
> This helper will be later used in patch #5.
> 
> Patch #5 changes multifd_send_sync_main() so it returns int instead of void.
> The return value is used to understand if any error happened in the function,
> allowing migration to possible fail earlier.
> 
> Patch #6 implements an workaround: The behavior introduced in d48c3a0445 is
> hard to deal with in zerocopy, so a workaround is introduced to send the
> header in a different syscall, without MSG_ZEROCOPY.
> 
> Patch #7 Makes use of QIOChannelSocket zero_copy implementation on
> nocomp multifd migration.

Queued.

> Results:
> In preliminary tests, the resource usage of __sys_sendmsg() reduced 15 times,
> and the overall migration took 13-22% less time, based in synthetic cpu
> workload.
> 
> In further tests, it was noted that, on multifd migration with 8 channels:
> - On idle hosts, migration time reduced in 10% to 21%.
> - On hosts busy with heavy cpu stress (1 stress thread per cpu, but
>   not cpu-pinned) migration time reduced in ~25% by enabling zero-copy.
> - On hosts with heavy cpu-pinned workloads (1 stress thread per cpu, 
>   cpu-pinned), migration time reducted in ~66% by enabling zero-copy.

Nice.

> Above tests setup:
> - Sending and Receiving hosts:
>   - CPU : Intel(R) Xeon(R) Platinum 8276L CPU @ 2.20GHz (448 CPUS)
>   - Network card: E810-C (100Gbps)
>   - >1TB RAM
>   - QEMU: Upstream master branch + This patchset
>   - Linux: Upstream v5.15 

That configuration is particularly interesting because while it's a big
machine with lots of cores, the individual cores are clocked relatively
slowly; also having lots of cores probably means they're all fighting
over memory bandwidth, so the less copies the better.

Dave

> - VM configuration:
>   - 28 VCPUs
>   - 512GB RAM
> 
> 
> ---
> Changes since v9:
> - Patch #6 got simplified and improved (thanks Daniel)
> - Patch #7 got better comments (thanks Peter Xu)
> 
> Changes since v8:
> - Inserted two new patches #5 & #6, previous patch #5 is now #7.
> - Workaround an optimization introduced in d48c3a0445
> - Removed unnecessary assert in qio_channel_writev_full_all
> 
> Changes since v7:
> - Migration property renamed from zero-copy to zero-copy-send
> - A few early tests added to help misconfigurations to fail earlier
> - qio_channel_full*_flags() renamed back to qio_channel_full*()
> - multifd_send_sync_main() reverted back to not receiving a flag,
>   so it always sync zero-copy when enabled.
> - Improve code quality on a few points
> 
> Changes since v6:
> - Remove io_writev_zero_copy(), and makes use of io_writev() new flags
>   to achieve the same results.
> - Rename io_flush_zero_copy() to io_flush()
> - Previous patch #2 became too small, so it was squashed in previous
>   patch #3 (now patch #2)
> 
> Changes since v5:
> - flush_zero_copy now returns -1 on fail, 0 on success, and 1 when all
>   processed writes were not able to use zerocopy in kernel.
> - qio_channel_socket_poll() removed, using qio_channel_wait() instead
> - ENOBUFS is now processed inside qio_channel_socket_writev_flags()
> - Most zerocopy parameter validation moved to migrate_params_check(),
>   leaving only feature test to socket_outgoing_migration() callback
> - Naming went from *zerocopy to *zero_copy or *zero-copy, due to QAPI/QMP
>   preferences
> - Improved docs
> 
> Changes since v4:
> - 3 patches got splitted in 6
> - Flush is used for syncing after each iteration, instead of only at the end
> - If zerocopy is not available, fail in connect instead of failing on write
> - 'multifd-zerocopy' property renamed to 'zerocopy'
> - Fail migrations that don't support zerocopy, if it's enabled.
> - Instead of checking for zerocopy at each write, save the flags in
>   MultiFDSendParams->write_flags and use them on write
> - Reorganized flag usage in QIOChannelSocket 
> - A lot of typos fixed
> - More doc on buffer restrictions
> 
> Changes since v3:
> - QIOChannel interface names changed from io_async_{writev,flush} to
>   io_{writev,flush}_zerocopy
> - Instead of falling back in case zerocopy is not implemented, return
>   error and abort operation.
> - Flush now waits as long as needed, or return error in case anything
>   goes wrong, aborting the operation.
> - Zerocopy is now conditional in multifd, being set by parameter
>   multifd-zerocopy
> - Moves zerocopy_flush to multifd_send_sync_main() from multifd_save_cleanup
>   so migration can abort if flush goes wrong.
> - Several other small improvements
> 
> Changes since v2:
> - Patch #1: One more fallback
> - Patch #2: Fall back to sync if fails to lock buffer memory in MSG_ZEROCOPY 
> send.
> 
> Changes since v1:
> - Reimplemented the patchset using async_write + async_flush approach.
> - Implemented a flush to be able to tell whenever all data was written.
> 
> Leonardo Bras (7):
>   QIOChannel: Add flags on io_writev and introduce io_flush callback
>   QIOChannelSocket: Implement io_writev zero copy flag & io_flush for
>     CONFIG_LINUX
>   migration: Add zero-copy-send parameter for QMP/HMP for Linux
>   migration: Add migrate_use_tls() helper
>   multifd: multifd_send_sync_main now returns negative on error
>   multifd: Send header packet without flags if zero-copy-send is enabled
>   multifd: Implement zero copy write in multifd migration
>     (multifd-zero-copy)
> 
>  qapi/migration.json                 |  24 ++++++
>  include/io/channel-socket.h         |   2 +
>  include/io/channel.h                |  38 +++++++++-
>  migration/migration.h               |   6 ++
>  migration/multifd.h                 |   4 +-
>  chardev/char-io.c                   |   2 +-
>  hw/remote/mpqemu-link.c             |   2 +-
>  io/channel-buffer.c                 |   1 +
>  io/channel-command.c                |   1 +
>  io/channel-file.c                   |   1 +
>  io/channel-socket.c                 | 110 +++++++++++++++++++++++++++-
>  io/channel-tls.c                    |   1 +
>  io/channel-websock.c                |   1 +
>  io/channel.c                        |  49 ++++++++++---
>  migration/channel.c                 |   3 +-
>  migration/migration.c               |  52 ++++++++++++-
>  migration/multifd.c                 |  75 +++++++++++++++----
>  migration/ram.c                     |  29 ++++++--
>  migration/rdma.c                    |   1 +
>  migration/socket.c                  |  12 ++-
>  monitor/hmp-cmds.c                  |   6 ++
>  scsi/pr-manager-helper.c            |   2 +-
>  tests/unit/test-io-channel-socket.c |   1 +
>  23 files changed, 379 insertions(+), 44 deletions(-)
> 
> -- 
> 2.36.0
> 
> From c6fda6f8fb29ceeaf4d36f3787b85196e9bb281f Mon Sep 17 00:00:00 2001
> From: Leonardo Bras <leobras@redhat.com>
> Date: Mon, 25 Apr 2022 17:45:14 -0300
> MIME-Version: 1.0
> Content-Type: text/plain; charset=UTF-8
> Content-Transfer-Encoding: 8bit
> To: "Marc-André Lureau" <marcandre.lureau@redhat.com>,Paolo Bonzini 
> <pbonzini@redhat.com>,Elena Ufimtseva 
> <elena.ufimtseva@oracle.com>,Jagannathan Raman <jag.raman@oracle.com>,John G 
> Johnson <john.g.johnson@oracle.com>,"Daniel P. Berrangé" 
> <berrange@redhat.com>,Juan Quintela <quintela@redhat.com>,"Dr. David Alan 
> Gilbert" <dgilbert@redhat.com>,Eric Blake <eblake@redhat.com>,Markus 
> Armbruster <armbru@redhat.com>,Fam Zheng <fam@euphon.net>,Peter Xu 
> <peterx@redhat.com>
> Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org, 
> 
> 
> Leonardo Bras (7):
>   QIOChannel: Add flags on io_writev and introduce io_flush callback
>   QIOChannelSocket: Implement io_writev zero copy flag & io_flush for
>     CONFIG_LINUX
>   migration: Add zero-copy-send parameter for QMP/HMP for Linux
>   migration: Add migrate_use_tls() helper
>   multifd: multifd_send_sync_main now returns negative on error
>   multifd: Send header packet without flags if zero-copy-send is enabled
>   multifd: Implement zero copy write in multifd migration
>     (multifd-zero-copy)
> 
>  qapi/migration.json                 |  24 ++++++
>  include/io/channel-socket.h         |   2 +
>  include/io/channel.h                |  38 +++++++++-
>  migration/migration.h               |   6 ++
>  migration/multifd.h                 |   4 +-
>  chardev/char-io.c                   |   2 +-
>  hw/remote/mpqemu-link.c             |   2 +-
>  io/channel-buffer.c                 |   1 +
>  io/channel-command.c                |   1 +
>  io/channel-file.c                   |   1 +
>  io/channel-socket.c                 | 110 +++++++++++++++++++++++++++-
>  io/channel-tls.c                    |   1 +
>  io/channel-websock.c                |   1 +
>  io/channel.c                        |  49 ++++++++++---
>  migration/channel.c                 |   3 +-
>  migration/migration.c               |  52 ++++++++++++-
>  migration/multifd.c                 |  72 +++++++++++++++---
>  migration/ram.c                     |  29 ++++++--
>  migration/rdma.c                    |   1 +
>  migration/socket.c                  |  12 ++-
>  monitor/hmp-cmds.c                  |   6 ++
>  scsi/pr-manager-helper.c            |   2 +-
>  tests/unit/test-io-channel-socket.c |   1 +
>  23 files changed, 377 insertions(+), 43 deletions(-)
> 
> -- 
> 2.36.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK