poll system call timeout

Question

Attaching strace shows a lot of these messages:

poll([{fd=5, events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=10, events=POLLIN}], 6, 0) = 0 (Timeout)

poll([{fd=5, events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=10, events=POLLIN}], 6, 0) = 0 (Timeout)

poll([{fd=5, events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=10, events=POLLIN}], 6, 0) = 0 (Timeout)

poll([{fd=5, events=POLLIN}, {fd=6, events=POLLIN}, {fd=7, events=POLLIN}, {fd=8, events=POLLIN}, {fd=9, events=POLLIN}, {fd=10, events=POLLIN}], 6, 0) = 0 (Timeout)

How can I find what file the program is trying to access that causes poll system call to timeout?

strace generates a lot of messages which make it hard to debug

Shriek

Depends. What are these six fds? Did you find matching calls to open() in the trace log? – Apocynaceous

I don't know. WHat does that mean? – Shriek

Can you explain the steps in a reply? What do you mean by matching? – Shriek

Assuming these file descriptors are actually files (as opposed to sockets or pipes), there should be calls to the open() syscall in the output of strace. You would have to look for the ones that returned 5, 6, etc. in order to determine which files were opened. – Apocynaceous

Answer 1

How can I find what file the program is trying to access that causes poll system call to timeout?

Invoke:

lsof -p <pid>

And see what the file descriptors in question refer to.

You can also take a look into proc filesystem on Linux:

ls -l /proc/<pid>/fd

Overburdensome

Here it happens when I try to # su - vuXXXX which has worked before. Can it be that su tries to lookup a host name or so? – Ischium

I stopped dbus and tried it again. And it went very fast. So the bottleneck/culprit is dbus in my case. – Ischium

I found the actual culprit: elogind Session Management caused the slowdown. – Ischium

Answer 2

ls -l /proc//fd/ -> You will get the corresponding node Id. ss -p | grep -> you will get the socket file path the fd belongs to.

Expenditure