Date: Mon, 21 Oct 2013 09:39:46 -0700 From: Colin Percival <cperciva@freebsd.org> To: Andre Oppermann <andre@freebsd.org>, Julian Elischer <julian@freebsd.org>, freebsd-net@freebsd.org Subject: Re: LRO causing stretch ACK violations interacts badly with delayed ACKing Message-ID: <526558D2.3010505@freebsd.org> In-Reply-To: <5265450C.1060601@freebsd.org> References: <52605EC9.6090406@freebsd.org> <526478D0.1000601@freebsd.org> <5264869E.4000308@freebsd.org> <5265450C.1060601@freebsd.org>
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On 10/21/13 08:15, Andre Oppermann wrote:
> On 21.10.2013 03:42, Colin Percival wrote:
>> I can't find any changes in netfront.c or tcp_lro.c to explain why 9.1 and
>> 9.2 are behaving differently -- anyone have any ideas?
>
> The last time I looked our soft-LRO had a few remaining issues. One of
> them was that in certain situations reordering may happen with segments
> that can't be aggregated into a LRO state. The other was that the driver
> is responsible to manage the flushing of LRO states that haven't seen
> updates in some time.
It looks like the netfront driver flushes LRO every time it finishes reading
packets -- if anything, it's too aggressive:
/*
* Flush any outstanding LRO work
*/
while (!SLIST_EMPTY(&lro->lro_active)) {
queued = SLIST_FIRST(&lro->lro_active);
SLIST_REMOVE_HEAD(&lro->lro_active, next);
tcp_lro_flush(lro, queued);
}
So unless that code is broken somehow (it looks reasonable to me) I don't think
it's a problem of data getting stuck in soft-LRO.
Looking at the TCP stack on the other hand confuses me -- I see code which seems
to be saying that we can delay-ACK any time that we're receiving data and don't
have a delayed ACK already pending, without any regard for the fact that we
might be receiving 2+ MSS at once... am I missing something here?
--
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid
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