linux-kernel - Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

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Date:	Mon, 15 Jun 2009 21:43:44 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, mingo@...hat.com,
	hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	penberg@...helsinki.fi, vegard.nossum@...il.com, efault@....de,
	jeremy@...p.org, npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > If it's faster, this becomes a legit (albeit complex) 
> > micro-optimization in a _very_ hot codepath.
> 
> I don't think it's all that hot. It's not like it's the return to 
> user mode.

Well i guess it depends. For server apps it is true - syscalls are a 
lot more dominant, MMs are long-running so any startup cost gets 
amortized and pagefaults are avoided.

For something like a kernel build we have 7 times as many pagefaults 
as syscalls:

aldebaran:~/linux/linux> perf stat -- make -j32 >/dev/null
[...]
 Performance counter stats for 'make -j32':

 1444281.076741  task-clock-msecs     #     14.429 CPUs 
         219991  context-switches     #      0.000 M/sec
          18335  CPU-migrations       #      0.000 M/sec
       38465628  page-faults          #      0.027 M/sec
  4374762924204  cycles               #   3029.025 M/sec
  2645979309823  instructions         #      0.605 IPC  
    42398991227  cache-references     #     29.356 M/sec
     4371920878  cache-misses         #      3.027 M/sec

  100.097787566  seconds time elapsed.

So we have 38465628 page-faults, or one every 68788 instructions, 
one every 113731 cycles.

10 cycles saved in the page fault costs means 0.01% performance win 
- or about 10 milliseconds shaven off the kernel build time.
 
100 cycles saved (which is impossible really in the entry/exit path) 
would mean 0.1% win.

5653639 syscalls (according to strace -c) - which is a factor of 6.8 
lower. Same goes for shell scripts or most of the clicking we do on 
a GUI.

It's not a big factor for sure.

Btw., the biggest pagefault cost is in the fault handling itself 
(the page clearing):

      4.14%  [k] do_page_fault
      1.20%  [k] sys_write
      1.10%  [k] sys_open
      0.63%  [k] sys_exit_group
      0.48%  [k] smp_apic_timer_interrupt
      0.37%  [k] sys_read
      0.37%  [k] sys_execve
      0.20%  [k] sys_mmap
      0.18%  [k] sys_close
      0.14%  [k] sys_munmap
      0.13%  [k] sys_poll
      0.09%  [k] sys_newstat
      0.07%  [k] sys_clone
      0.06%  [k] sys_newfstat

it totals to 4.14% of the total cost (user-space cycles included) of 
a kernel build, on a Nehalem box.

	Ingo
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