lkml.org 
[lkml]   [2015]   [Apr]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
Subject[Question] How does perf still record the stack of a specified pid even when that process is interrupted and CPU is scheduled to other process
[Profiling Background]
Now we are profiling the performance of ext4 and f2fs on an eMMC card with iozone,
we find a case that ext4 is better than f2fs in random write under the test of
"iozone -s 262144 -r 64 -i 0 -i 2". We want to analyze the I/O delay of the two
file systems. We have got a conclusion that 1% of sys_write takes up 60% time of
the overall sys_write (262144/64=4096). We want to find out the call stack during
this specific 1% sys_write. Our idea is to record the stack in a certain time period
and since the specific 1% case takes up 60% time, the total number of records of its
stack should also takes up 60% of the total records, then we can recognize those stacks
and figure out what the I/O stack of f2fs is doing in the 1% case.

[Profiling Problem]

Although perf can record the events (with call stack) of a specified pid, e.g. using
"perf record -g iozone -s 262144 -r 64 -i 0 -i 2". But we find iozone is interrupted
and the CPU is scheduled to other process. As a result, perf will not record any events
of iozone until iozone's context is recovered and the CPU is scheduled to continue
processing the sys_write of iozone. This obeys our initial idea which is described in
[Profiling Background], since we cannot recognize the call stacks of the specific 1% case
by using the ratio of the record number.

[Alternative Solution without Perf]
We can use /proc/#pid/stack to record the stack in a certain period (e.g. 1ms) of iozone,
no matter whether iozone is interrupted or not. However, we have not taken a deep sight
into this, since we want to use perf to do this kind of thing.

[Question about Perf]
So we have a question that "How does perf still record the stack of a specified pid even
when that process is interrupted and CPU is scheduled to other process?"

--
Thanks,
Yunlong Song



\
 
 \ /
  Last update: 2015-04-24 16:01    [W:0.098 / U:0.208 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site