| There were essentialy three things we had to do for such a drastic drop
1) recompile CEPH —without-tcmalloc 2) pin the OSDs to a set of a specific NUMA zone - we had this for a long time and it really helped 3) migrate the OSD memory to the correct CPU with migratepages - we will use cgroups in the future for this, should make life easier and is the only correct solution
It is similiar to the effect of just restarting the OSD, but much better - since we immediately see hundreds of connections on a freshly restarted OSD (and in the benchmark the tcmalloc issue manifested with just two clients in parallel) I’d say we never saw the raw performance with tcmalloc (undegraded), but it was never this good - consistently low latencies, much smaller spikes when something happens and much lower CPU usage (about 50% savings but we’re also backfilling a lot on the background). Workloads are faster as well - like reweighting OSDs on that same node was much (hundreds of percent) faster.
So far the effect has been drastic. I wonder why tcmalloc was even used when people are having problems with it? The glibc malloc seems to work just fine for us.
The only concerning thing is the virtual memory usage - we are over 400GB VSS with a few OSDs. That doesn’t hurt anything, though.
Jan
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Did you see what the effect of just restarting the OSDs before using tcmalloc? I've noticed that there is usually a good drop for us just by restarting them. I don't think it is usually this drastic.
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Robert LeBlanc
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On Wed, Jun 24, 2015 at 2:08 AM, Jan Schermer wrote:
Can you guess when we did that?
Still on dumpling, btw...
http://www.zviratko.net/link/notcmalloc.png
Jan
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