when execute pt-pmp -p pid, the mysqld's tps/qps will drop a lot

[liuhao]

We use the pt-pmp tool a lot to analyze our online MySQL problem.

But when we execute the pmp, we have found tps/qps of mysqld drop a lot, such as:

/usr/bin/pt-pmp -p 18266

[10225s] threads: 256, tps: 8738.19, reads/s: 0.00, writes/s: 8735.39
[10230s] threads: 256, tps: 8749.81, reads/s: 0.00, writes/s: 8745.21
[10235s] threads: 256, tps: 8757.80, reads/s: 0.00, writes/s: 8763.00
[10240s] threads: 256, tps: 8772.80, reads/s: 0.00, writes/s: 8766.40
[10245s] threads: 256, tps: 284.80, reads/s: 0.00, writes/s: 278.40
[10250s] threads: 256, tps: 8036.65, reads/s: 0.00, writes/s: 8038.85
[10255s] threads: 256, tps: 8772.41, reads/s: 0.00, writes/s: 8773.01

We modify the code of pt-pmp to fix it.

The patch is below:

143,148d142
< if gdb -nx --quiet --batch --readnever > /dev/null 2>&1; then
< readnever=--readnever
< else
< readnever=
< fi
<
205c199
< gdb $readnever -ex "set pagination 0" -ex "thread apply all bt" -batch -p $OPT_p >> "${OPT_k:-$TMPDIR/percona-toolkit}"
---
> gdb -ex "set pagination 0" -ex "thread apply all bt" -batch -p $OPT_p >> "${OPT_k:-$TMPDIR/percona-toolkit}"

[Hui Liu (hickey)]

The qps drop is due to gdb attaching/dumping thread symbols.

It's a good suggestion to add "--readnever" option for GDB, just like pstack script.

[Onacorpuscle (onacorpuscle)]

Thank you for the workaround!
Regards.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PT-1226