Systemtap caching behavior

[Mikhail Zhukovskiy <mikhail dot zhukovskiy at oracle dot com>]

Hello,

I have a question about systemtap caching behavior.

According to the man page, the results of pass 3 and pass 4 (the translation to C and the compilation of the kernel module) should be cached between runs, however this is not what I am observing. Could you please point out what I’m doing wrong?

We are attempting to debug 1-2 minute long load spikes, with an stap script triggered when a load spike is detected. If the results of the compilation steps are not cached and compilation is necessary on every stap invocation, the event may be over before data capture begins.

I am attaching my invocation of stap and a demonstration that all compilation steps re-occur on subsequent runs.

Thank you very much for your help!

Mike

vagrant@dns4-01-us1:~/.systemtap$ sudo rm -rf ~/.systemtap

vagrant@dns4-01-us1:~$ cat flame-kernel.sh
#!/bin/bash

 /home/vagrant/systemtapbin/bin/stap -k -s 32 --all-modules -v -D MAXBACKTRACE=100 -D MAXSTRINGLEN=4096 -D MAXMAPENTRIES=10240 -D DMAXACTION_INTERRUPTIBLE=500 -D MAXACTION=100000 -D STP_OVERLOAD_THRESHOLD=5000000000LL flame-kernel.stp  > out.stap-stacks

vagrant@dns4-01-us1:~$ sudo ./flame-kernel.sh
Pass 1: parsed user script and 476 library scripts using 113060virt/85440res/4612shr/81280data kb, in 350usr/20sys/377real ms.
Pass 2: analyzed script: 3 probes, 7 functions, 4 embeds, 1 global using 250420virt/224400res/6408shr/218640data kb, in 1900usr/300sys/3033real ms.
Pass 3: translated to C into "/tmp/stapy6Mjkr/stap_11482_src.c" using 250420virt/224584res/6536shr/218640data kb, in 1230usr/70sys/1304real ms.
Pass 4: compiled C into "stap_11482.ko" in 16830usr/1230sys/18702real ms.
Pass 5: starting run.
WARNING: Missing unwind data for a module, rerun with 'stap -d kernel'
WARNING: Missing unwind data for a module, rerun with 'stap -d (unknown; retry with -DDEBUG_UNWIND)'
Pass 5: run completed in 10usr/200sys/30546real ms.
Keeping temporary directory "/tmp/stapy6Mjkr"

vagrant@dns4-01-us1:~$ find ~/.systemtap -name *.ko
/home/vagrant/.systemtap/cache/b4/typequery_b43a87dffeccfe937c15e5ca0b8cccf7_874.ko
/home/vagrant/.systemtap/cache/ea/typequery_ea54c2c3c9c4e050b149b086326979b2_877.ko


vagrant@dns4-01-us1:~$ sudo ./flame-kernel.sh
Pass 1: parsed user script and 476 library scripts using 113060virt/85560res/4732shr/81280data kb, in 330usr/40sys/371real ms.
Pass 2: analyzed script: 3 probes, 7 functions, 4 embeds, 1 global using 250408virt/224372res/6392shr/218628data kb, in 790usr/210sys/998real ms.
Pass 3: translated to C into "/tmp/stapEXDMjx/stap_13078_src.c" using 250408virt/224556res/6520shr/218628data kb, in 1220usr/70sys/1299real ms.
Pass 4: compiled C into "stap_13078.ko" in 8790usr/360sys/8817real ms.
Pass 5: starting run.
WARNING: Missing unwind data for a module, rerun with 'stap -d kernel'
Pass 5: run completed in 10usr/200sys/30549real ms.
Keeping temporary directory "/tmp/stapEXDMjx"
vagrant@dns4-01-us1:~$