Re: [PATCH] Btrfs: allocate the free space by the existed max extent size when ENOSPC

[David Sterba <dsterba@suse.cz>]

On Thu, Aug 29, 2013 at 01:47:38PM +0800, Miao Xie wrote:
> By the current code, if the requested size is very large, and all the extents
> in the free space cache are small, we will waste lots of the cpu time to cut
> the requested size in half and search the cache again and again until it gets
> down to the size the allocator can return. In fact, we can know the max extent
> size in the cache after the first search, so we needn't cut the size in half
> repeatedly, and just use the max extent size directly. This way can save
> lots of cpu time and make the performance grow up when there are only fragments
> in the free space cache.
> 
> According to my test, if there are only 4KB free space extents in the fs,
> and the total size of those extents are 256MB, we can reduce the execute
> time of the following test from 5.4s to 1.4s.
>   dd if=/dev/zero of=<testfile> bs=1MB count=1 oflag=sync

Sounds like a good improvement! Can you please post the test? Fragmented
free space is nothing uncommon, so I guess aging the filesystem for a
while works as well and there the improvement would show up in reduced
cpu time.

Patch looks ok.

david