From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Samuel Zhang <guoqing.zhang@amd.com>
Cc: qemu-devel@nongnu.org, peterx@redhat.com, farosas@suse.de,
lizhijian@fujitsu.com, eblake@redhat.com, armbru@redhat.com,
Emily.Deng@amd.com, Victor.Zhao@amd.com, PengJu.Zhou@amd.com,
Qing.Ma@amd.com
Subject: Re: [PATCH v6] migration/rdma: add x-rdma-chunk-size parameter
Date: Mon, 27 Apr 2026 08:17:25 +0100 [thread overview]
Message-ID: <ae8NhTr69D7Wzejz@redhat.com> (raw)
In-Reply-To: <20260427031401.3895523-1-guoqing.zhang@amd.com>
On Mon, Apr 27, 2026 at 11:14:01AM +0800, Samuel Zhang wrote:
> The default 1MB RDMA chunk size causes slow live migration because
> each chunk triggers a write_flush (ibv_post_send). For 8GB RAM,
> 1MB chunk size produces ~15000 flushes vs ~3700 with 1024MB chunk size.
>
> Add x-rdma-chunk-size parameter to configure the RDMA chunk size for
> faster migration.
> Usage: `migrate_set_parameter x-rdma-chunk-size 1024M`
>
> Performance with RDMA live migration of 8GB RAM VM:
>
> | x-rdma-chunk-size (B) | time (s) | throughput (MB/s) |
> |-----------------------|----------|-------------------|
> | 1M (default) | 37.915 | 1,007 |
> | 32M | 17.880 | 2,260 |
> | 1024M | 4.368 | 17,529 |
What is the downside of setting a larger chunk size ?
IOW, why should we keep 1M as the default when it gives
such terrible relative performance ? Why not make 1G
be the default instead of creating this flag and requiring
people to know about setting it ?
next prev parent reply other threads:[~2026-04-27 7:18 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-27 3:14 [PATCH v6] migration/rdma: add x-rdma-chunk-size parameter Samuel Zhang
2026-04-27 7:17 ` Daniel P. Berrangé [this message]
2026-04-30 9:46 ` Zhang, GuoQing (Sam)
2026-04-30 10:36 ` Daniel P. Berrangé
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=ae8NhTr69D7Wzejz@redhat.com \
--to=berrange@redhat.com \
--cc=Emily.Deng@amd.com \
--cc=PengJu.Zhou@amd.com \
--cc=Qing.Ma@amd.com \
--cc=Victor.Zhao@amd.com \
--cc=armbru@redhat.com \
--cc=eblake@redhat.com \
--cc=farosas@suse.de \
--cc=guoqing.zhang@amd.com \
--cc=lizhijian@fujitsu.com \
--cc=peterx@redhat.com \
--cc=qemu-devel@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.