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POGetting user-space stack information from perf
 

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    <p>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.</p> <p>I thought Linux's <code>perf</code> 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.</p> <p>It's easy to record block I/O requests and completions with, eg:</p> <pre><code>sudo perf record -g -T -u postgres -e 'block:block_rq_*' </code></pre> <p>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 <code>perf script</code> output like:</p> <pre><code>postgres 7462 [002] 301125.113632: block:block_rq_issue: 8,0 W 0 () 208078848 + 1024 [postgres] </code></pre> <p>If I add the <code>-g</code> flag to <code>perf record</code> it'll take snapshots of the <em>kernel</em> 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 <code>LWLockRelease</code>, <code>LWLockAcquire</code>, <code>memcpy</code> (mmap'd IO), <code>__GI___libc_write</code>, etc.</p> <p>So. Any tips? Being able to capture a snapshot of the <em>user-space</em> stack in response to <em>kernel</em> events would be ideal.</p> <p>I'm on Fedora 19, 3.11.3-201.fc19.x86_64, Schrödinger’s Cat, with perf version 3.10.9-200.fc19.x86_64.</p>
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