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SubjectRe: [PATCH v3 next/akpm] aio: convert the ioctx list to radix tree
On Mon, 15 Apr 2013 14:40:55 +0300 Octavian Purdila <octavian.purdila@intel.com> wrote:

> When using a large number of threads performing AIO operations the
> IOCTX list may get a significant number of entries which will cause
> significant overhead. For example, when running this fio script:
>
> rw=randrw; size=256k ;directory=/mnt/fio; ioengine=libaio; iodepth=1
> blocksize=1024; numjobs=512; thread; loops=100
>
> on an EXT2 filesystem mounted on top of a ramdisk we can observe up to
> 30% CPU time spent by lookup_ioctx:
>
> 32.51% [guest.kernel] [g] lookup_ioctx
> 9.19% [guest.kernel] [g] __lock_acquire.isra.28
> 4.40% [guest.kernel] [g] lock_release
> 4.19% [guest.kernel] [g] sched_clock_local
> 3.86% [guest.kernel] [g] local_clock
> 3.68% [guest.kernel] [g] native_sched_clock
> 3.08% [guest.kernel] [g] sched_clock_cpu
> 2.64% [guest.kernel] [g] lock_release_holdtime.part.11
> 2.60% [guest.kernel] [g] memcpy
> 2.33% [guest.kernel] [g] lock_acquired
> 2.25% [guest.kernel] [g] lock_acquire
> 1.84% [guest.kernel] [g] do_io_submit
>
> This patchs converts the ioctx list to a radix tree.

The patch looks nice. One thing we should pay attention to is the
memory consumption. radix-trees can be far less space-efficient than
lists, and as the tree key comes from mmap() it can be pretty sparsely
distributed.

So could you please have a think about this, see if we can cook up some
worst-case numbers and decide if they are problematic?



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