[PATCH] fix up sysctl_tcp_mem initialization

[PATCH] fix up sysctl_tcp_mem initialization

[John Heffner]

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The initial values of sysctl_tcp_mem are sometimes greater than the 
total memory in the system (particularly on SMP systems).  This patch 
ensures that tcp_mem[2] is always <= 3/4 nr_kernel_pages.

However, I wonder if we want to set this differently than the way this 
patch does it.  Depending on how far off the memory size is from a power 
of two (exactly equal to a power of two is the worst case), and if total 
memory <128M, it can be substantially less than 3/4.

   -John

[-- Attachment #2: tcp_mem_init.patch --]
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Fix up tcp_mem initiail settings to take into account the size of the
hash entries (different on SMP and non-SMP systems).

Signed-off-by: John Heffner <jheffner@psc.edu>

---
commit d4ef8c8245c0a033622ce9ba9e25d379475254f6
tree 5377b8af0bac3b92161188e7369a84e472b5acb2
parent ea55b7c31b47edf90132baea9a088da3bbe2bb5c
author John Heffner <jheffner@psc.edu> Tue, 14 Nov 2006 14:53:27 -0500
committer John Heffner <jheffner@psc.edu> Tue, 14 Nov 2006 14:53:27 -0500

 net/ipv4/tcp.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 4322318..c05e8ed 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2316,9 +2316,10 @@ void __init tcp_init(void)
 		sysctl_max_syn_backlog = 128;
 	}
 
-	sysctl_tcp_mem[0] =  768 << order;
-	sysctl_tcp_mem[1] = 1024 << order;
-	sysctl_tcp_mem[2] = 1536 << order;
+	/* Allow no more than 3/4 kernel memory (usually less) allocated to TCP */
+	sysctl_tcp_mem[0] = (1536 / sizeof (struct inet_bind_hashbucket)) << order;
+	sysctl_tcp_mem[1] = sysctl_tcp_mem[0] * 4 / 3;
+	sysctl_tcp_mem[2] = sysctl_tcp_mem[0] * 2;
 
 	limit = ((unsigned long)sysctl_tcp_mem[1]) << (PAGE_SHIFT - 7);
 	max_share = min(4UL*1024*1024, limit);

Re: [PATCH] fix up sysctl_tcp_mem initialization

[David Miller]

From: John Heffner <jheffner@psc.edu>
Date: Tue, 14 Nov 2006 15:02:05 -0500

> The initial values of sysctl_tcp_mem are sometimes greater than the 
> total memory in the system (particularly on SMP systems).  This patch 
> ensures that tcp_mem[2] is always <= 3/4 nr_kernel_pages.

This is a genuine bug fix so I'll apply this, thanks for taking
care of these corner-case bugs in TCP sizing Jeff.

> However, I wonder if we want to set this differently than the way this 
> patch does it.  Depending on how far off the memory size is from a power 
> of two (exactly equal to a power of two is the worst case), and if total 
> memory <128M, it can be substantially less than 3/4.

Longer term, yes, probably a better way exists.

So you concern is that when we round to a power of 2 like we do
now, we often mis-shoot?

Re: [PATCH] fix up sysctl_tcp_mem initialization

[John Heffner]

David Miller wrote:
>> However, I wonder if we want to set this differently than the way this 
>> patch does it.  Depending on how far off the memory size is from a power 
>> of two (exactly equal to a power of two is the worst case), and if total 
>> memory <128M, it can be substantially less than 3/4.
> 
> Longer term, yes, probably a better way exists.
> 
> So you concern is that when we round to a power of 2 like we do
> now, we often mis-shoot?

I'm not that concerned about it, but basically yes, there are big (x2) 
jumps on power-of-two memory size boundaries.  There's also a bigger 
(x8) discontinuity at 128k pages.  It could be smoother.

   -John