On Fri, May 10, 2013 at 01:40:15PM -0700, Andrew Morton wrote: > On Mon, 15 Apr 2013 14:40:55 +0300 Octavian Purdila > <octavian.purd...@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?
Because the overhead of an ioctx is so high (ringbuffer is some number of pages) it shouldn't matter much - but I wouldn't mind seeing a bit of arithmatic. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
