Work (really slow directory access on ext4)

[Nick Krause]

On Fri, Jul 25, 2014 at 9:08 PM, Arlie Stephens <arlie at worldash.org> wrote:
> On Jul 25 2014, Valdis.Kletnieks at vt.edu wrote:
>> On Fri, 25 Jul 2014 15:23:42 -0700, Arlie Stephens said:
>>
>> > If you want an annoying problem, explain and/or fix directory
>> > performance on ext4. I've got a server where an ls of a directory took
>> > 5 seconds, according to "time", even though it only has 295 entries at
>> > present.
>>
>> I don't suppose you could get a trace of where that ls is spending its
>> time with the kernel's trace facilities, or even just getting a stack trace
>> of where that ls is in the kernel?
>
> These are all very good questions.
>
> To my amazement, I found that no one had yet fixed the problem by
> deleting and recreating the directory, and I do have sudo access.
> This time it was only 4 seconds...
>      real 0m3.992s
>      user 0m0.005s
>      sys  0m0.052s
>
>> I'll go out on a limb and ask if a *second* ls of the same directory runs
>> quickly because it's now cache-hot.  If so, I'd start looking at whether
>> there's large amounts of *other* disk activity going on, and the reads of the
>> directory are getting hung in the I/O queue behind other disk
>> read/writes.
>
> Sure enough, the cache saved me on a second read -
>      real 0m0.010s
>      user 0m0.000s
>      sys  0m0.010s
>
>> Also, are you doing an 'ls' (which just requires reading the name/inode#
>> pairs), or an 'ls -l' whihc in addition requires a stat() call to read in the
>> inode itself)?  That makes a lot of difference.  Cache-cold on my laptop, and a
>> *huge* Mail/linux-kernel directory (yes, it really *is* an 11M directory,
>> it's got a half-million entries in it):
>
> I was doing a vanilla ls. So was the original reporter, unless he has
> some really strange aliases.
>
>
> I'm afraid I'll be rather unpopular if I drop the caches on the system
> in question, creating a burst of poor performance, so my best bet is
> probably to see what I can do with ftrace on Monday, or perhaps
> partway through the weekend.
>
> There is normally a fair amount of disk activity going on - much of it
> writes. So I can expect cached blocks to age out in a reasonable time.
>
>
>> [~] echo 3 >| /proc/sys/vm/drop_caches
>> [~] cd Mail
>> [~/Mail] time ls linux-kernel/ | wc -l
>> 478401
>>
>> real    0m2.387s
>> user    0m0.500s
>> sys     0m0.433s
>> [~/Mail] ls -ld linux-kernel/
>> drwxr-xr-x. 2 valdis valdis 11005952 Jul 25 19:30 linux-kernel/
>
> Compared to your directory, mine is microscopic
>
> $ ls -ld xxxx
> drwxr-xr-x 2 yyy yyy 36864 Jul 25 12:19 xxxx
>
>
>> [~/Mail] time ls -l linux-kernel/ | wc -l
>> 478402
>>
>> real    0m32.915s
>> user    0m2.483s
>> sys     0m20.787s
>
> --
> Arlie
>
> (Arlie Stephens                                 arlie at worldash.org)


Arlie,
Whenever you get around to it is fine.
Just send me a log.
Cheers Nick