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From Will Hayworth <whaywo...@atlassian.com>
Subject Back to the futex()? :(
Date Sat, 06 Feb 2016 23:28:47 GMT
*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>

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