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Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy
From: |
Peter Xu |
Subject: |
Re: [PATCH v1 3/3] migration: multifd: Enable zerocopy |
Date: |
Tue, 31 Aug 2021 16:29:09 -0400 |
On Tue, Aug 31, 2021 at 02:16:42PM +0100, Daniel P. Berrangé wrote:
> On Tue, Aug 31, 2021 at 08:02:39AM -0300, Leonardo Bras wrote:
> > Call qio_channel_set_zerocopy(true) in the start of every multifd thread.
> >
> > Change the send_write() interface of multifd, allowing it to pass down
> > flags for qio_channel_write*().
> >
> > Pass down MSG_ZEROCOPY flag for sending memory pages, while keeping the
> > other data being sent at the default copying approach.
> >
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> > migration/multifd-zlib.c | 7 ++++---
> > migration/multifd-zstd.c | 7 ++++---
> > migration/multifd.c | 9 ++++++---
> > migration/multifd.h | 3 ++-
> > 4 files changed, 16 insertions(+), 10 deletions(-)
>
> > @@ -675,7 +676,8 @@ static void *multifd_send_thread(void *opaque)
> > }
> >
> > if (used) {
> > - ret = multifd_send_state->ops->send_write(p, used,
> > &local_err);
> > + ret = multifd_send_state->ops->send_write(p, used,
> > MSG_ZEROCOPY,
> > + &local_err);
>
> I don't think it is valid to unconditionally enable this feature due to the
> resource usage implications
>
> https://www.kernel.org/doc/html/v5.4/networking/msg_zerocopy.html
>
> "A zerocopy failure will return -1 with errno ENOBUFS. This happens
> if the socket option was not set, the socket exceeds its optmem
> limit or the user exceeds its ulimit on locked pages."
>
> The limit on locked pages is something that looks very likely to be
> exceeded unless you happen to be running a QEMU config that already
> implies locked memory (eg PCI assignment)
Yes it would be great to be a migration capability in parallel to multifd. At
initial phase if it's easy to be implemented on multi-fd only, we can add a
dependency between the caps. In the future we can remove that dependency when
the code is ready to go without multifd. Thanks,
--
Peter Xu