Additionally: this isn't the futex_wait bug (or at least it shouldn't be?). Amazon says <https://forums.aws.amazon.com/thread.jspa?messageID=623731> that was fixed several kernel versions before mine, which is 4.1.10-17.31.amzn1.x86_64. And the reason my heap is so large is because, per CASSANDRA-9472, we can't use offheap until 3.4 is released.
Will ___________________________________________________________ Will Hayworth Developer, Engagement Engine Atlassian My pronoun is "they". <http://pronoun.is/they> On Sat, Feb 6, 2016 at 3:28 PM, Will Hayworth <whaywo...@atlassian.com> wrote: > *tl;dr: other than CAS operations, what are the potential sources of lock > contention in C*?* > > Hi all! :) I'm a novice Cassandra and Linux admin who's been preparing a > small cluster for production, and I've been seeing something weird. For > background: I'm running 3.2.1 on a cluster of 12 EC2 m4.2xlarges (32 GB > RAM, 8 HT cores) backed by 3.5 TB GP2 EBS volumes. Until late yesterday, > that was a cluster of 12 m4.xlarges with 3 TB volumes. I bumped it because > while backloading historical data I had been seeing awful throughput (20K > op/s at CL.ONE). I'd read through Al Tobey's *amazing* C* tuning guide > <https://tobert.github.io/pages/als-cassandra-21-tuning-guide.html> once > or twice before but this time I was careful and fixed a bunch of defaults > that just weren't right, in cassandra.yaml/JVM options/block device > parameters. Folks on IRC were super helpful as always (hat tip to Jeff > Jirsa in particular) and pointed out, for example, that I shouldn't be > using DTCS for loading historical data--heh. After changing to LTCS, > unbatching my writes* and reserving a CPU core for interrupts and fixing > the clocksource to TSC, I finally hit 80K early this morning. Hooray! :) > > Now, my question: I'm still seeing a *ton* of blocked processes in the > vmstats, anything from 2 to 9 per 10 second sample period--and this is > before EBS is even being hit! I've been trying in vain to figure out what > this could be--GC seems very quiet, after all. On Al's page's advice, I've > been running strace and, indeed, I've been seeing *tens of thousands of > futex() calls* in periods of 10 or 20 seconds. What eludes me is *where* this > lock contention is coming from. I'm not using LWTs or performing CAS > operations of which I'm aware. Assuming this isn't a red herring, what > gives? > > Sorry for the essay--I just wanted to err on the side of more context--and > *thank > you* for any advice you'd like to offer, > Will > > P.S. More background if you'd like--I'm running on Amazon Linux 2015.09, > using jemalloc 3.6, JDK 1.8.0_65-b17. Here <http://pastebin.com/kuhBmHXG> is > my cassandra.yaml and here <http://pastebin.com/fyXeTfRa> are my JVM > args. I realized I neglected to adjust memtable_flush_writers as I was > writing this--so I'll get on that. Aside from that, I'm not sure what to > do. (Thanks, again, for reading.) > > * They were batched for consistency--I'm hoping to return to using them > when I'm back at normal load, which is tiny compared to backloading, but > the impact on performance was eye-opening. > ___________________________________________________________ > Will Hayworth > Developer, Engagement Engine > Atlassian > > My pronoun is "they". <http://pronoun.is/they> > > >
