{{parent page="Linux"}}

===strace===
strace is an important utility to query system call usage.

==tracing a running process==
%%
strace -p $PID
%%

==Get a process's system call category report==
Use **strace -c**

%%
strace -p 3712 -c
Process 3712 attached - interrupt to quit
Process 3712 detached
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 94.27    0.006843           3      2641           poll
  4.16    0.000302           0      7781           gettimeofday
  1.20    0.000087           1        70           futex
  0.37    0.000027           0      5244           ioctl
  0.00    0.000000           0       510           read
  0.00    0.000000           0       233           write
------ ----------- ----------- --------- --------- ----------------
100.00    0.007259                 16479           total
%%

==Tracing a hung process==

Do a strace -p on the process, if it ends with 
%%
read(13,
%%

That means the system is hung on a read system call, to the file descriptor 13. Do a **lsof** to find out what fd #13 is.