Re: [PATCH 1/1] nbd/server: push pending frames after sending reply

From: Kevin Wolf <kwolf@redhat.com>
To: Florian Westphal <fw@strlen.de>
Cc: qemu-block@nongnu.org, eblake@redhat.com,
	vsementsov@yandex-team.ru, qemu-devel@nongnu.org
Subject: Re: [PATCH 1/1] nbd/server: push pending frames after sending reply
Date: Mon, 27 Mar 2023 13:44:59 +0200	[thread overview]
Message-ID: <ZCGBu4ZVzecC0qJy@redhat.com> (raw)
In-Reply-To: <20230324104720.2498-1-fw@strlen.de>

Am 24.03.2023 um 11:47 hat Florian Westphal geschrieben:
> qemu-nbd doesn't set TCP_NODELAY on the tcp socket.
> 
> Kernel waits for more data and avoids transmission of small packets.
> Without TLS this is barely noticeable, but with TLS this really shows.
> 
> Booting a VM via qemu-nbd on localhost (with tls) takes more than
> 2 minutes on my system.  tcpdump shows frequent wait periods, where no
> packets get sent for a 40ms period.
> 
> Add explicit (un)corking when processing (and responding to) requests.
> "TCP_CORK, &zero" after earlier "CORK, &one" will flush pending data.
> 
> VM Boot time:
> main:    no tls:  23s, with tls: 2m45s
> patched: no tls:  14s, with tls: 15s
> 
> VM Boot time, qemu-nbd via network (same lan):
> main:    no tls:  18s, with tls: 1m50s
> patched: no tls:  17s, with tls: 18s
> 
> Future optimization: if we could detect if there is another pending
> request we could defer the uncork operation because more data would be
> appended.
> 
> Signed-off-by: Florian Westphal <fw@strlen.de>

Thanks, applied to the block branch.

Kevin