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SubjectRe: [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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