linux-kernel - Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

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Message-ID: <20081117195757.GA12020@elte.hu>
Date:	Mon, 17 Nov 2008 20:57:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Eric Dumazet <dada1@...mosbay.com>,
	David Miller <davem@...emloft.net>, rjw@...k.pl,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	cl@...ux-foundation.org, efault@....de, a.p.zijlstra@...llo.nl,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: [Bug #11308] tbench regression on each kernel release from
	2.6.22 -&gt; 2.6.28


> [ I'll post per function analysis as i complete them, as a reply to
>   this mail. ]

[ i'll do a separate mail for every function analyzed, the discussion 
  spreads better that way. ]

> 100.000000 total
> ................
>   7.253355 copy_user_generic_string

This is the Well-known pattern of user-copy overhead, which centers 
around this single REP MOVS instruction:

                nr-of-hits
                 .........
ffffffff80341eea:       42 	83 e2 07    		and    $0x7,%edx
ffffffff80341eed:   677398 	f3 48 a5         	rep movsq %ds:(%rsi),%es:(%rdi)
ffffffff80341ef0:     3642 	89 d1                	mov    %edx,%ecx
ffffffff80341ef2:    16260 	f3 a4                	rep movsb %ds:(%rsi),%es:(%rdi)
ffffffff80341ef4:     6554 	31 c0                	xor    %eax,%eax
ffffffff80341ef6:     1958 	c3                   	retq   
ffffffff80341ef7:        0 	90                   	nop    
ffffffff80341ef8:        0 	90                   	nop    

That's to be expected - tbench shuffles 3.5 GB of effective data 
to/from sockets. That's 7.5 GB due to double-copy. So for every 64 
bytes of data transferred we spend 1.4 CPU cycles in this specific 
function - that is OK-ish.

	Ingo
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