5

I have no strace skill and knowlegde, but im trying to work through a problem where my clients application is awfully slow. And im trying to find out where the bottleneck / problem may lie.

So I ran

/etc/init.d/apache2 stop && strace -Tf -o /tmp/trace.txt /etc/init.d/apache2 start

And going through /tmp/trace.txt, im seeing quite a lot of the following. 

2540  poll([{fd=21, events=POLLIN|POLLPRI}], 1, 0) = 0 (Timeout) <0.000052>

Would anyone please what im seeing and, and how would I go about tying it to where the problem is. The site does use MySQL, does this mean the connection to MySQL could not be established? Is the poll even DB related.

Kyle Brandt
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Brent
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1 Answers1

5

Take a step back first -- you are getting very low level before finding out where the problem generally is. A simple way to do this is to see if static HTTP pages are slow from Apache -- if they are not then maybe the DB is slow. The next step I would take is to see how long DB queries are taking.

Also if this is all on the same system you can look at system resources will tools such as top, iotop, and iostat.

Regarding the poll system call:

I would try adding the -c switch to the strace of Apache to see what your timings are on each syscall if you narrow it down to apache before focusing on a system call. For example the top of my output from a healthy HAProxy is:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 38.79    0.001152           0      6089           epoll_wait

The poll system call is used to wait for for an available an event on a file descriptor. My first guess since the timeout is so low that this is the normal functioning of Apache. It might be that Apache is running out of file descriptors since a file descriptor is needed for each network socket if this actually a problem. You could look at Apache manual section on File Descriptors if you get to this point. But according to DerkK "it'd be failing at open() or socket()" which makes a lot more sense.

So really this is just some detail on what very likely isn't your actual problem -- again take several steps back.

Kyle Brandt
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