netdev - Re: TCP rx window autotuning harmful at LAN context

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Date:	Wed, 11 Mar 2009 11:03:35 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	David Miller <davem@...emloft.net>
Cc:	md@....sk, netdev@...r.kernel.org
Subject: Re: TCP rx window autotuning harmful at LAN context

David Miller <davem@...emloft.net> writes:

> From: Marian ÄurkoviÄ <md@....sk>
> Date: Mon, 9 Mar 2009 21:05:05 +0100
>
>> Well, in practice that was always limited by receive window size, which
>> was by default 64 kB on most operating systems. So this undesirable behavior
>> was limited to hosts where receive window was manually increased to huge
>> values.
>
> You say "was" as if this was a recent change.  Linux has been doing
> receive buffer autotuning for at least 5 years if not longer.

I think his point was the only now does it become a visible problem
as >= 1GB of memory is wide spread, which leads to 4MB rx buffer sizes.

Perhaps this points to the default buffer sizing heuristics to 
be too aggressive for >= 1GB?

Perhaps something like this patch? Marian, does that help?

-Andi

TCP: Lower per socket RX buffer sizing threshold 

Signed-off-by: Andi Kleen <ak@...ux.intel.com>

---
 net/ipv4/tcp.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

Index: linux-2.6.28-test/net/ipv4/tcp.c
===================================================================
--- linux-2.6.28-test.orig/net/ipv4/tcp.c	2009-02-09 11:06:52.000000000 +0100
+++ linux-2.6.28-test/net/ipv4/tcp.c	2009-03-11 11:01:53.000000000 +0100
@@ -2757,9 +2757,9 @@
 	sysctl_tcp_mem[1] = limit;
 	sysctl_tcp_mem[2] = sysctl_tcp_mem[0] * 2;
 
-	/* Set per-socket limits to no more than 1/128 the pressure threshold */
-	limit = ((unsigned long)sysctl_tcp_mem[1]) << (PAGE_SHIFT - 7);
-	max_share = min(4UL*1024*1024, limit);
+	/* Set per-socket limits to no more than 1/256 the pressure threshold */
+	limit = ((unsigned long)sysctl_tcp_mem[1]) << (PAGE_SHIFT - 8);
+	max_share = min(2UL*1024*1024, limit);
 
 	sysctl_tcp_wmem[0] = SK_MEM_QUANTUM;
 	sysctl_tcp_wmem[1] = 16*1024;


-- 
ak@...ux.intel.com -- Speaking for myself only.
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