Over the last few days, I’ve spent a bunch of time improving Pacemaker’s performance in large clusters.
This involved profiling the CIB and Policy Engine, identifying and optimizing hotspots and improving algorithm designs.
Since most of my work is done in virtual machines, it wasn’t possible to use oprofile.
Strictly speaking oprofile worked, but without hardware performance counters the results weren’t very helpful. I also tried gprof, but that is more about counting calls rather than time spent.
Eventually I switched to callgrind and when combined with a tool Tim found called Gprof2Dot and/or kcachegrind, finally got the data I was looking for.
To do your own profiling, simply set PCMK_callgrind_enabled to either yes or to the name of a Pacemaker daemon you wish to profile. Eg. PCMK_callgrind_enabled=cib
Overall, the CIB (which is the main bottleneck in a large cluster) and the Policy Engine are about 70% faster.
The improvements will be available with 1.1.4 is released next month, or from our 1.1 code repository right now.
A summary of the various changes and description of future work is below.
Any assistance in further optimization would be appreciated :-)
— Andrew
PE
Use case:
* 100 nodes
* 100 clones, clone-max=100 (10,000 effective resources)
* 100 resource location constraints
Baseline: with probes 20-30 minutes
Baseline: without probes 28s
Phase 1
Use hashtables instead of lists for stores the available nodes for a resource
New time without probes: 18s
Phase 2
Defer creation of deletion,promote and demote constraints until they are needed
New time without probes: 13s
Phase 3
Use g_list_prepend() instead of g_list_append() for the list of ordering constraints
New time without probes: 5s
Phase 4
New algorithm for determining which clone instances need probing
New time with probes: 31s
Future work
- Further improve the algorithm for determining which resources need to be probed
- Further optimize the algorithm for enforcing ordering constraints
CIB
The CIB was harder to profile.
Rather than give it one large task to chew through and see how long it took using a few printf’s to provide granularity, I had to run it through a profiler while it was operating in a real cluster and see where most of the time was being spent.
Phase 1
Remove most uses of cib_msg_copy(), reduced the amount of needless copying.
Phase speedup: 10%
Phase 2
Compression costs a LOT, don’t do it unless we’re hitting message limits.
For now, use 256k as the threshold at which compression kicks in.
The previous limit was 10k, compressing 184 of 1071 messages accounted for 23% of the total CPU used by the cib.
Each time we validated the CIB, we were re-reading and re-parsing the RelaxNG schema, which accounted for 28% of the CIB’s CPU usage on the DC.
We now read it once and cache the result for the life of the CIB process.
Phase speedup: 51%
Phase 3
Push detection of group and set ordering changes to (the less busy) slave instances.
This detection was costing 15% of the CIB’s total CPU time on the DC.
Phase speedup: 15%
Future work
The majority of CPU spent by the CIB is in post-processing.
- Detecting what changed so we can minimize the network load: diff_xml_object, 35.5% CPU time
- Calculating the current digest so peers can verify the diffs and detect ordering changes: calculate_xml_digest, 31% CPU time
