From: Stefano Brivio <sbrivio@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: passt-dev@passt.top
Subject: Re: [PATCH 7/8] tcp_splice: Remove questionable "optimisation" of pending bytes tracking
Date: Thu, 04 Jun 2026 06:41:43 +0200 (CEST) [thread overview]
Message-ID: <20260604064142.5abdaced@elisabeth> (raw)
In-Reply-To: <20260528050213.679685-8-david@gibson.dropbear.id.au>
On Thu, 28 May 2026 15:02:12 +1000
David Gibson <david@gibson.dropbear.id.au> wrote:
> We have a special path that avoids updating conn->pending when the amounts
> read and written are equal. This has a conceptual complexity cost, in
> particular, it means that conn->pending[] is not accurate to its normal
> meaning for a section of the loop body.
>
> conn->pending[] shares a cacheline with conn->pipe[] and conn->s[], so it's
> almost certainly cache-hot. It's questionable that avoiding the update
> of pending even outweighs the extra conditional branch, let alone saves
> anything of significance. Remove it.
I added this when we still had 64-bit counters, that is, two weeks
before commit 37c228ada88b ("tap, tcp, udp, icmp: Cut down on some
oversized buffers"), but now, as you point out, it doesn't make sense anymore.
--
Stefano
next prev parent reply other threads:[~2026-06-04 4:41 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-28 5:02 [PATCH 0/8] splice() forwarding cleanups David Gibson
2026-05-28 5:02 ` [PATCH 1/8] tcp_splice: Remove never-invoked SO_RCVLOWAT logic David Gibson
2026-05-28 5:02 ` [PATCH 2/8] tcp_splice: Simplify EPOLLRDHUP / eof / FIN handling David Gibson
2026-05-28 5:02 ` [PATCH 3/8] tcp_splice: Improve EOF exit condition for the loop David Gibson
2026-05-28 5:02 ` [PATCH 4/8] tcp_splice: Remove goto from forwarding loop David Gibson
2026-05-28 5:02 ` [PATCH 5/8] tcp_splice: Simplify shutdown(2) handling David Gibson
2026-05-28 5:02 ` [PATCH 6/8] tcp_splice: Simplify / correct OUT_WAIT flag handling David Gibson
2026-06-04 4:41 ` Stefano Brivio
2026-06-04 5:14 ` David Gibson
2026-05-28 5:02 ` [PATCH 7/8] tcp_splice: Remove questionable "optimisation" of pending bytes tracking David Gibson
2026-06-04 4:41 ` Stefano Brivio [this message]
2026-06-04 5:14 ` David Gibson
2026-05-28 5:02 ` [PATCH 8/8] tcp_splice: Exit forwarding earlier when stalled read side David Gibson
2026-06-04 4:41 ` Stefano Brivio
2026-06-04 5:26 ` David Gibson
2026-06-04 5:44 ` Stefano Brivio
2026-06-04 7:08 ` David Gibson
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