Tracking down origin of I/O in multi-process server
- by Craig Ringer
I'm currently trying to track down some phantom I/O in a PostgreSQL build I'm testing. It's a multi-process server and it isn't simple to associate disk I/O back to a particular back-end and query.
I thought Linux's perf tool would be ideal for this, but I'm struggling to capture block I/O performance counter metrics and associate them with user-space activity.
It's easy to record block I/O requests and completions with, eg:
sudo perf record -g -T -u postgres -e 'block:block_rq_*'
and the user-space pid is recorded, but there's no kernel or user-space stack captured, or ability to snapshot bits of the user-space process's heap (say, query text) etc. So while you have the pid, you don't know what the process was doing at that point. Just perf script output like:
postgres 7462 [002] 301125.113632: block:block_rq_issue: 8,0 W 0 () 208078848 + 1024 [postgres]
If I add the -g flag to perf record it'll take snapshots of the kernel stack, but doesn't capture user-space state for perf events captured in the kernel. The user-space stack only goes up to the entry-point from userspace, like LWLockRelease, LWLockAcquire, memcpy (mmap'd IO), __GI___libc_write, etc.
So. Any tips? Being able to capture a snapshot of the user-space stack in response to kernel events would be ideal.
