Modify SST code to use fork()/exec() to allow cleanup on fatal signals

Bug #1382797 reported by Alexey Kopytov
This bug affects 5 people
Affects Status Importance Assigned to Milestone
Percona XtraDB Cluster moved to
Status tracked in 5.6
Fix Released
Alexey Kopytov
Fix Released
Alexey Kopytov

Bug Description

The SST code in uses posix_spawnp() to spawn a shell process executing the actual SST script. I don't know if there's any viable reason to use posix_spawnp() instead of fork()/exec(), but one problem we can solve by doing it with fork()/exec() is cleaning up children (i.e. processes spawned by the SST script) when mysqld crashes or is killed with a non-interceptable signal such as SIGKILL.

See comment #4 in

Related branches

description: updated
description: updated
Revision history for this message
Alex Yurchenko (ayurchen) wrote :

Alexey, the reason posix_spawn() is used to fork SST script is that at that time it seemed to be the only way to spawn a process without creating a full copy of the parent memory image. On dedicated servers where mysqld used more than 50% of RAM and with no swap space, forking SST script used to fail due to memory constraints.

I suppose that the problem with cleaning children arises from the not-entirely-correct usage of posix_spawn() in SST code. Perhaps passing proper flags would have solved it.

Revision history for this message
Alexey Kopytov (akopytov) wrote :

OK, thanks for clarifications.

The reason posix_spawn() is faster and consumes less RAM is that it is implemented via vfork() / exec(), when the POSIX_SPAWN_USEVFORK attribute is used, which is what the SST code passes to posix_spawnattr_setflags().

I'm not sure if we can do anything about cleaning up children with the current code. For that, we need to make sure the _child_ SST process calls prctl(PR_SET_DEATHSIG, ...) as the first thing after it starts up. I don't see a way to achieve that with posix_spawn().

Howeven, we can "emulate" posix_spawn() with vfork()/exec() without the perfromance/resource penalty imposed by regular fork().
We can then call prctl(PR_SET_DEATHSIG, ...) in the child before we call exec().

Changed in percona-xtradb-cluster:
status: New → Confirmed
Revision history for this message
Shahriyar Rzayev (rzayev-sehriyar) wrote :

Percona now uses JIRA for bug reports so this bug report is migrated to:

To post a comment you must log in.
This report contains Public information  Edit
Everyone can see this information.

Duplicates of this bug

Other bug subscribers