Date:	Wed, 29 Feb 2012 23:44:31 -0500
From:	Theodore Tso <tytso@....EDU>
To:	Jacek Luczak <difrost.kernel@...il.com>
Cc:	Theodore Tso <tytso@....edu>, linux-ext4@...r.kernel.org,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, linux-btrfs@...r.kernel.org
Subject: Re: getdents - ext4 vs btrfs performance

You might try sorting the entries returned by readdir by inode number before you stat them.    This is a long-standing weakness in ext3/ext4, and it has to do with how we added hashed tree indexes to directories in (a) a backwards compatible way, that (b) was POSIX compliant with respect to adding and removing directory entries concurrently with reading all of the directory entries using readdir.

You might try compiling spd_readdir from the e2fsprogs source tree (in the contrib directory):

http://git.kernel.org/?p=fs/ext2/e2fsprogs.git;a=blob;f=contrib/spd_readdir.c;h=f89832cd7146a6f5313162255f057c5a754a4b84;hb=d9a5d37535794842358e1cfe4faa4a89804ed209

… and then using that as a LD_PRELOAD, and see how that changes things.

The short version is that we can't easily do this in the kernel since it's a problem that primarily shows up with very big directories, and using non-swappable kernel memory to store all of the directory entries and then sort them so they can be returned in inode number just isn't practical.   It is something which can be easily done in userspace, though, and a number of programs (including mutt for its Maildir support) does do, and it helps greatly for workloads where you are calling readdir() followed by something that needs to access the inode (i.e., stat, unlink, etc.)

-- Ted


On Feb 29, 2012, at 8:52 AM, Jacek Luczak wrote:

> Hi All,
> 
> /*Sorry for sending incomplete email, hit wrong button :) I guess I
> can't use Gmail */
> 
> Long story short: We've found that operations on a directory structure
> holding many dirs takes ages on ext4.
> 
> The Question: Why there's that huge difference in ext4 and btrfs? See
> below test results for real values.
> 
> Background: I had to backup a Jenkins directory holding workspace for
> few projects which were co from svn (implies lot of extra .svn dirs).
> The copy takes lot of time (at least more than I've expected) and
> process was mostly in D (disk sleep). I've dig more and done some
> extra test to see if this is not a regression on block/fs site. To
> isolate the issue I've also performed same tests on btrfs.
> 
> Test environment configuration:
> 1) HW: HP ProLiant BL460 G6, 48 GB of memory, 2x 6 core Intel X5670 HT
> enabled, Smart Array P410i, RAID 1 on top of 2x 10K RPM SAS HDDs.
> 2) Kernels: All tests were done on following kernels:
> - 2.6.39.4-3 -- the build ID (3) is used here for internal tacking of
> config changes mostly. In -3 we've introduced ,,fix readahead pipeline
> break caused by block plug'' patch. Otherwise it's pure 2.6.39.4.
> - 3.2.7 -- latest kernel at the time of testing (3.2.8 has been
> release recently).
> 3) A subject of tests, directory holding:
> - 54GB of data (measured on ext4)
> - 1978149 files
> - 844008 directories
> 4) Mount options:
> - ext4 -- errors=remount-ro,noatime,
> data=writeback
> - btrfs -- noatime,nodatacow and for later investigation on
> copression effect: noatime,nodatacow,compress=lzo
> 
> In all tests I've been measuring time of execution. Following tests
> were performed:
> - find . -type d
> - find . -type f
> - cp -a
> - rm -rf
> 
> Ext4 results:
> | Type     | 2.6.39.4-3   | 3.2.7
> | Dir cnt  | 17m 40sec  | 11m 20sec
> | File cnt |  17m 36sec | 11m 22sec
> | Copy    | 1h 28m        | 1h 27m
> | Remove| 3m 43sec    | 3m 38sec
> 
> Btrfs results (without lzo comression):
> | Type     | 2.6.39.4-3   | 3.2.7
> | Dir cnt  | 2m 22sec  | 2m 21sec
> | File cnt |  2m 26sec | 2m 23sec
> | Copy    | 36m 22sec | 39m 35sec
> | Remove| 7m 51sec   | 10m 43sec
> 
> From above one can see that copy takes close to 1h less on btrfs. I've
> done strace counting times of calls, results are as follows (from
> 3.2.7):
> 1) Ext4 (only to elements):
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
> 57.01   13.257850           1  15082163           read
> 23.40    5.440353           3   1687702           getdents
> 6.15    1.430559           0   3672418           lstat
> 3.80    0.883767           0  13106961           write
> 2.32    0.539959           0   4794099           open
> 1.69    0.393589           0    843695           mkdir
> 1.28    0.296700           0   5637802           setxattr
> 0.80    0.186539           0   7325195           stat
> 
> 2) Btrfs:
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
> 53.38    9.486210           1  15179751           read
> 11.38    2.021662           1   1688328           getdents
> 10.64    1.890234           0   4800317           open
> 6.83    1.213723           0  13201590           write
> 4.85    0.862731           0   5644314           setxattr
> 3.50    0.621194           1    844008           mkdir
> 2.75    0.489059           0   3675992         1 lstat
> 1.71    0.303544           0   5644314           llistxattr
> 1.50    0.265943           0   1978149           utimes
> 1.02    0.180585           0   5644314    844008 getxattr
> 
> On btrfs getdents takes much less time which prove the bottleneck in
> copy time on ext4 is this syscall. In 2.6.39.4 it shows even less time
> for getdents:
> % time     seconds  usecs/call     calls    errors syscall
> ------ ----------- ----------- --------- --------- ----------------
> 50.77   10.978816           1  15033132           read
> 14.46    3.125996           1   4733589           open
> 7.15    1.546311           0   5566988           setxattr
> 5.89    1.273845           0   3626505           lstat
> 5.81    1.255858           1   1667050           getdents
> 5.66    1.224403           0  13083022           write
> 3.40    0.735114           1    833371           mkdir
> 1.96    0.424881           0   5566988           llistxattr
> 
> 
> Why so huge difference in the getdents timings?
> 
> -Jacek
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