2000x performance win

I recently helped analyze a performance issue in an unexpected but common place, where the fix improved performance of a task by around 2000x (two thousand times faster). As this is short, interesting and useful, I’ve reproduced it here in a lab environment to share details and screenshots.

Issue

In a production SmartOS cloud environment, a script is used to count entries in an active log. This script is executed from cron(1M), and the grep(1) command it uses is running very slowly indeed:

# time grep done log | wc -l
1492751

real    8m56.062s
user    8m12.275s
sys     0m0.218s
# ls -l log
-rw-r--r-- 1 root root 18M 2011-12-08 21:59 log

That took almost nine minutes. The grep(1) command is processing this file at about 34 Kbytes/sec, which seems awfully slow. Dropping the “wc -l” and using “grep -c” instead didn’t make much difference.

Hypothesis

I have a method for attacking performance issues (which I’ll explain in a later article); it starts with checking for errors, because they do cause performance issues and are a fast area to check.

The slow time sounds like it could be disk I/O related: either slow reads caused by disk errors, or random disk reads. I first checked the error counts:

# iostat -En | grep Hard
sd0              Soft Errors: 0 Hard Errors: 0 Transport Errors: 0
sd1              Soft Errors: 0 Hard Errors: 0 Transport Errors: 0

Which were fine, and then for random reads:

# iostat -xnz 1
                    extended device statistics
    r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
    0.2    0.3    1.3    1.0  0.0  0.0    0.0    0.0   0   0 ramdisk1
   12.3   81.6  355.4 3005.9  0.0  0.1    0.0    0.7   0   2 sd1
                    extended device statistics
    r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
    0.0    7.0    0.0  245.7  0.0  0.0    0.0    0.1   0   0 sd1
                    extended device statistics
    r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
                    extended device statistics
    r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
    0.0    4.0    0.0   59.5  0.0  0.0    0.0    0.1   0   0 sd1
[...]

Which are not present. This doesn’t look disk related.

Analysis

A great way to see what threads are doing is with “prstat -mLc 1″, which uses thread microstate accounting (only available on Solaris-related systems):

# prstat -mLc 1
[...]
   PID USERNAME USR SYS TRP TFL DFL LCK SLP LAT VCX ICX SCL SIG PROCESS/LWPID
 14566 root     100 0.0 0.0 0.0 0.0 0.0 0.0 0.0   0  13   0   0 app1/1
 12740 root      98 0.0 0.0 0.0 0.0 0.0 0.0 2.1   1  26   8   0 grep/1
 18998 root     3.0  12 0.0 0.0 0.0 0.0  85 0.0  45   0  5K   0 app2/1
 17347 root     0.1 0.2 0.0 0.0 0.0 0.0 100 0.0  16   0 170   0 prstat/1
 29693 root     0.0 0.0 0.0 0.0 0.0 0.0 100 0.0  13   0 128   0 sshd/1
 12741 root     0.0 0.0 0.0 0.0 0.0 0.0 100 0.0   7   0   7   0 wc/1
[...]

The output shows grep(1) spending 98% of its time in USR (running in user-land), and 2.1% in LAT (CPU dispatcher queue latency – ready to run). There is no high percentage of SLP (sleep) time for disk I/O, which we already suspected from using iostat(1) earlier. (This lab system is also running a couple of dummy applications to simulate production load.)

Hot CPU issues are some of the easiest to solve, usually through profiling. Here, I’ve used DTrace to sample grep(1) user-land stack traces at 997 Hertz. I ran this one-liner for several seconds, then hit Ctrl-C to stop sampling and print the report:

# dtrace -n 'profile-997 /execname == "grep"/ { @[ustack()] = count(); }'
dtrace: description 'profile-997 ' matched 1 probe
^C
[...]
              libc.so.1`_UTF8_mbrtowc
              grep`check_multibyte_string+0x85
              grep`EGexecute+0xa6
              grep`grepbuf+0xf2
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
              406

              libc.so.1`mbrtowc+0x1d
              grep`EGexecute+0xa6
              grep`grepbuf+0xf2
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
              498

              grep`check_multibyte_string+0xd5
              grep`EGexecute+0xa6
              grep`grepbuf+0xf2
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
             1023

This shows codepaths that were sampled. At the top is the on-CPU function, then its ancestory. The number is the count of samples for that stack trace.

The hot functions included check_multibyte_string(), mbrtowc() and _UTF8_mbrtowc(). check_multibyte_string() is declared in grep(1) (prefixed with “grep`”), and mbrtowc() is from libc. I’ll start with mbrtowc() – given that it’s in libc, it probably has a man page.

# man mbrtowc
mbrtowc(3C)              Standard C Library Functions              mbrtowc(3C)

NAME
       mbrtowc - convert a character to a wide-character code (restartable)
[...]

Wide character conversion?

Hypothesis #2

The team I was working with suggested this could be locale related:

# echo $LANG
en_US.UTF-8

I’d also punched “grep mbrtowc” into google and found grep – Bugs: bug #14472, grep is slow in multibyte locales [Savannah]“. But that’s the GNU version of grep(1), which we didn’t think we were using.

# which grep
/opt/local/bin/grep
# grep --version
GNU grep 2.5.3
[...]

Ah. So we are using GNU grep(1), and we have LANG set to en_US.UTF-8.

Experiment & Fix

# time LANG=C grep done log | wc -l
1492751

real    0m0.304s
user    0m0.361s
sys     0m0.057s

That’s over 1700x faster.

And using the SmartOS grep(1):

# time /usr/bin/grep done log | wc -l
1492751

real    0m0.268s
user    0m0.318s
sys     0m0.059s

That’s 2000x faster.

On an idle system, these improvements are between 100x – 200x (still huge). It seems that with production load, the long running grep(1) process takes even longer than usual. I’d suspect this was due in part to thread priorities and the kernel scheduler, giving the longer-running grep(1) a lower priority and less CPU time per-second than the short-lived grep(1). (If I needed to confirm this, I can examine the kernel scheduling events and priorities using DTrace.)

Cause

Both PATH and LANG had changed in the new systems, causing the GNU grep to be executed with a non-C locale. The environment was set is these locations:

# grep LANG= /etc/default/init
LANG=en_US.UTF-8
# grep PATH= ~/.profile 
MANPATH=/opt/local/man:/usr/share/man
PATH=/opt/local/bin:/opt/local/sbin:/usr/bin:/usr/sbin
# grep PATH= /etc/default/cron
PATH=/opt/local/bin:/opt/local/sbin:/usr/bin:/usr/sbin:/usr/sfw/bin
SUPATH=/opt/local/bin:/opt/local/sbin:/usr/bin:/usr/sbin:/usr/sfw/bin

Happy grep(1)

In case you are curious, here’s what GNU grep is doing when LANG=C, also profiled with DTrace:

[...]
              grep`prtext+0x230
              grep`grepbuf+0x51
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
                7

              grep`EGexecute+0x4e5
              grep`grepbuf+0xf2
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
                7

              libc.so.1`mutex_lock_impl+0xf8
              libc.so.1`mutex_lock+0xc
              libc.so.1`cancel_safe_mutex_lock+0x9
              libc.so.1`_flockget+0x1b
              libc.so.1`fwrite+0x67
              grep`prline+0x39a
              grep`prtext+0x21d
              grep`grepbuf+0x51
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
               15

              libc.so.1`clear_lockbyte+0x2c
              libc.so.1`mutex_unlock_queue+0x43
              libc.so.1`mutex_unlock+0x12c
              libc.so.1`cancel_safe_mutex_unlock+0x1f
              libc.so.1`fwrite+0xa2
              grep`prline+0x39a
              grep`prtext+0x21d
              grep`grepbuf+0x51
              grep`grepfile+0x4f3
              grep`main+0x1157
              grep`_start+0x6c
               18

Much lower sample counts (since it ended faster), and no mbrtowc(). The hottest two stacks show lock activity (which is where I’d start if I wanted to improve GNU grep(1) performance further).

A bit faster

Using LANG=C and grep(1)’s inbuilt “-c” to do counting instead of “wc -l”, makes it run even faster. Usually faster than 0.2s, and as fast as 0.12s:

# time LANG=C grep -c done log
1492751

real    0m0.120s
user    0m0.110s
sys     0m0.010s

Over 4000x.

Conclusion

Picking a non-C locale can hurt performance – something that has been known for many years. In this case, a GNU grep(1) bug inflated the translation overhead to slow down performance by a huge degree: up to 2000x. In short:

Leave LANG=C.

Aim DTrace at everything – even grep(1).

Posted on December 9, 2011 at 12:57 am by Brendan Gregg · Permalink
In: DTrace, performance, smartos