Re: [parisc-linux] Improving performance of munmap

[Carlos O'Donell <carlos@baldric.uwo.ca>]

On Thu, Oct 07, 2004 at 04:02:32PM -0700, Randolph Chung wrote:
> Hi all,
> 
> On a 2.6 SMP kernel, munmap()ing 16MB currently takes on the order of
> 0.8-0.9 seconds on a 440MHz machine. on a UP machine it takes about 5ms.
> 
> profiling shows that we are spending a lot of time flushing caches:
> 
> tausq@ios:~/parisc/linux-2.6$ sort -nr -k3 ~/test/mmap/after |head
>  48586 cpu_idle                                 759.1562
>  37906 machine_restart                          592.2812
>  10951 flush_user_icache_range_asm              304.1944
>  10946 flush_user_dcache_range_asm              304.0556
>    129 flush_kernel_icache_page                   1.2900
>     29 _spin_unlock_bh                            0.6042
>      9 fdsync                                     0.4500
>     10 _spin_unlock_irq                           0.4167
> 
> i believe this is related to the way we implement flush_tlb_mm(). on SMP
> we currently flush the entire tlb, instead of just invaliding the
> process context. this is needed to get the correct behavior if a
> multithreaded app is simultaneously running on >1 CPUs. James had
> suggested previously that we might be able to do something smarter, such
> as sending an ipi to the other CPU to switch the context. 
> 
> would anybody like to look into this problem, or suggest some ways to
> tackle this?

Is munmap() speed really a big issue? Does userland see the benefits of
a faster flush_tlb_mm()?

Perhaps if you call dlclose() *a lot* you might notice.

Or if you create and destroy threads really quickly.

Aside from that, who else in userspace is a big munmap caller?

c.

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