Keep in mind that compaction in LCS can only run 1 compaction per level.
Even if it wants to run more compactions in L0 it might be blocked
because it is already running a compaction in L0.
BR
Jan
On 01/16/2016 01:26 AM, Sebastian Estevez wrote:
LCS is IO ontensive but CPU is also relevant.
On slower disks compaction may not be cpu bound.
If you aren't seeing more than one compaction thread at a time, I
suspect your system is not compaction bound.
all the best,
Sebastián
On Jan 15, 2016 7:20 PM, "Kai Wang" <dep...@gmail.com
<mailto:dep...@gmail.com>> wrote:
Sebastian,
Because I have this impression that LCS is IO intensive and it's
recommended only on SSDs. So I am curious to see how far it can
stress those SSDs. But it turns out the most expensive part about
LCS is not IO bound but CUP bound, or more precisely single core
speed bound. This is a little surprising.
Of course LCS is still superior in other aspects.
On Jan 15, 2016 6:34 PM, "Sebastian Estevez"
<sebastian.este...@datastax.com
<mailto:sebastian.este...@datastax.com>> wrote:
Correct.
Why are you concerned with the raw throughput, are you
accumulating pending compactions? Are you seeing high sstables
per read statistics?
all the best,
Sebastián
On Jan 15, 2016 6:18 PM, "Kai Wang" <dep...@gmail.com
<mailto:dep...@gmail.com>> wrote:
Jeff & Sebastian,
Thanks for the reply. There are 12 cores but in my case C*
only uses one core most of the time. *nodetool
compactionstats* shows there's only one compactor running.
I can see C* process only uses one core. So I guess I
should've asked the question more clearly:
1. Is ~25 M/s a reasonable compaction throughput for one core?
2. Is there any configuration that affects single core
compaction throughput?
3. Is concurrent_compactors the only option to parallelize
compaction? If so, I guess it's the compaction strategy
itself that decides when to parallelize and when to block
on one core. Then there's not much we can do here.
Thanks.
On Fri, Jan 15, 2016 at 5:23 PM, Jeff Jirsa
<jeff.ji...@crowdstrike.com
<mailto:jeff.ji...@crowdstrike.com>> wrote:
With SSDs, the typical recommendation is up to 0.8-1
compactor per core (depending on other load). How many
CPU cores do you have?
From: Kai Wang
Reply-To: "user@cassandra.apache.org
<mailto:user@cassandra.apache.org>"
Date: Friday, January 15, 2016 at 12:53 PM
To: "user@cassandra.apache.org
<mailto:user@cassandra.apache.org>"
Subject: compaction throughput
Hi,
I am trying to figure out the bottleneck of compaction
on my node. The node is CentOS 7 and has SSDs
installed. The table is configured to use LCS. Here is
my compaction related configs in cassandra.yaml:
compaction_throughput_mb_per_sec: 160
concurrent_compactors: 4
I insert about 10G of data and start observing compaction.
*nodetool compaction* shows most of time there is one
compaction. Sometimes there are 3-4 (I suppose this is
controlled by concurrent_compactors). During the
compaction, I see one CPU core is 100%. At that point,
disk IO is about 20-25 M/s write which is much lower
than the disk is capable of. Even when there are 4
compactions running, I see CPU go to +400% but disk IO
is still at 20-25M/s write. I use *nodetool
setcompactionthroughput 0* to disable the compaction
throttle but don't see any difference.
Does this mean compaction is CPU bound? If so 20M/s is
kinda low. Is there anyway to improve the throughput?
Thanks.
