linux-kernel - Re: [patch] speed up / fix the new generic semaphore code (fix AIM7 40% regression with 2.6.26-rc1)

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Message-ID: <alpine.LFD.1.10.0805081535130.2940@woody.linux-foundation.org>
Date:	Thu, 8 May 2008 15:55:01 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [patch] speed up / fix the new generic semaphore code (fix AIM7
 40% regression with 2.6.26-rc1)

On Thu, 8 May 2008, Ingo Molnar wrote:
>
>    2512  down <= lock_kernel <= opost <= write_chan <
>    2574  down <= lock_kernel <= write_chan <= tty_write <

Ok. tty write handling. Nasty. But not as nasty as the open/close code, 
perhaps, and maybe we'll get it fixed some day.

In fact, I thought we had fixed most of this already, but hey, I was 
clearly wrong. I assume Alan looks at it occasionally and groans. Alan?

> 
> some other interesting stats. Top wakeups sources:
> 
>   [...]
>    1301  default_wake_function <= __wake_up_common <= __wake_up <= n_tty_receive_buf <= pty_write <= write_chan <
>    2065  wake_up_state <= prepare_signal <= send_signal <= __group_send_sig_info <= group_send_sig_info <= __kill_pgrp_info <

Ok, signals being the top one, but that tty code is pretty high again.

> and here's a few seconds worth of NMI driven readprofile output:
> 
> 216021 sync_page                                3375.3281
> 391888 page_check_address                       1414.7581
> 962212 total                                      0.3039
> 
> system overhead is consistently 20% during this test.
> 
> the page_check_address() overhead is surprising - tons of rmap 
> contention? about 10% wall-clock overhead in that function alone - and 
> this is just on a dual-core box!

No, it's not rmap contention. Your profile hits are just on the actual 
calculations, and it's all data-dependent arithmetic and loads. Some cache 
misses on the page tables, clearly, but it looks like a lot of it is even 
just the plain arithmetic (the imul followed by a data-dependent 'lea' 
instruction).

Some of it is that "page_to_pfn(page)", which involves a nasty division 
(divide by sizeof(struct page)). It gets turned into that shift and 
multiply, but it's still quite expensive with big constants etc.

			Linus
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