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From Arindam Barua <aba...@247-inc.com>
Subject RE: Config changes to leverage new hardware
Date Wed, 20 Nov 2013 23:42:24 GMT

Thanks for the suggestions Aaron.

As a follow up, we ran a bunch of tests with different combinations of these changes on a
2-node ring. The load was generated using cassandra-stress, run with default values to write
30 million rows, and read them back.
However, for both writes and reads there was virtually no difference in the latencies.

The different combinations attempted:

1.       Baseline test with none of the below changes.

2.       Grabbing the TLAB setting from 1.2

3.       Moving the commit logs too to the 7 disk RAID 0.

4.       Increasing the concurrent_read to 32, and concurrent_write to 64

5.       (3) + (4), i.e. moving commit logs to the RAID + increasing concurrent_read and concurrent_write
config to 32 and 64.

The write latencies were very similar, except them being ~3x worse for the 99.9th percentile
and above for scenario (5) above.
The read latencies were also similar, with (3) and (5) being a little worse for the 99.99th
percentile.

Overall, not making any changes, i.e. (1) performed as well or slightly better than any of
the other changes.

Running cassandra-stress on both the old and new hardware without making any config changes,
the write performance was very similar, but the new hardware did show ~10x improvement in
the read for the 99.9th percentile and higher. After thinking about this, the reason why we
were not seeing any difference with our test framework was perhaps the nature of the test
where we write the rows, and then do a bunch of reads to read the rows that were just written
immediately following. The data is read back from the memtables, and never from the disk/sstables.
Hence the new hardware's increased RAM and size of the disk cache or higher number of disks
never helps.

I'm still not very sure where the current *write* bottleneck is though. The new hardware has
32 cores vs 8 cores of the old hardware. Moving the commit log from a dedicated disk to a
7 RAID-0 disk system (where it would be shared by other data though) didn't make a difference
too. (unless the extra contention on the RAID nullified the positive effects of the RAID).

Sample iostat data (captured every 10s) for the dedicated disk where commit logs are written
is below. Does this seem like a bottle neck? When the commit logs are written the await/svctm
ratio is high.

Device:         rrqm/s   wrqm/s   r/s   w/s    rMB/s    wMB/s avgrq-sz avgqu-sz   await  svctm
 %util

               0.00     8.09  0.04  8.85     0.00     0.07    15.74     0.00    0.12   0.03
  0.02
               0.00   768.03  0.00  9.49     0.00     3.04   655.41     0.04    4.52   0.33
  0.31

               0.00     8.10  0.04  8.85     0.00     0.07    15.75     0.00    0.12   0.03
  0.02

               0.00   752.65  0.00 10.09     0.00     2.98   604.75     0.03    3.00   0.26
  0.26

Another interesting thing is that the linux disk cache doesn't seem to be growing in spite
of a lot of free memory available. The total disk cache used reported by 'free' is less than
the size of the sstables written with over 100 GB unused RAM.
Even in production, where we have the older hardware running with 32 GB RAM for a long time
now, looking at 5 hosts in 1 DC, only 2.5 GB to 8 GB was used for the disk cache. The Cassandra
java process uses the 8 GB allocated to it, and at least 10-15 GB on all the hosts is not
used at all.

Thanks,
Arindam

From: Aaron Morton [mailto:aaron@thelastpickle.com]
Sent: Wednesday, November 06, 2013 8:34 PM
To: Cassandra User
Subject: Re: Config changes to leverage new hardware

Running Cassandra 1.1.5 currently, but evaluating to upgrade to 1.2.11 soon.
You will make more use of the extra memory moving to 1.2 as it moves bloom filters and compression
data off heap.

Also grab the TLAB setting from cassandra-env.sh in v1.2

As of now, our performance tests (our application specific as well as cassandra-stress) are
not showing any significant difference in the hardwares, which is a little disheartening,
since the new hardware has a lot more RAM and CPU.
For reads or writes or both ?

Writes tend to scale with cores as long as the commit log can keep up.
Reads improve with disk IO and page cache size when the hot set is in memory.

Old Hardware: 8 cores (2 quad core), 32 GB RAM, four 1-TB disks ( 1 disk used for commitlog
and 3 disks RAID 0 for data)
New Hardware: 32 cores (2 8-core with hyperthreading), 128 GB RAM, eight 1-TB disks ( 1 disk
used for commitlog and 7 disks RAID 0 for data)
Is the disk IO on the commit log volume keeping up ?
You cranked up the concurrent writers and the commit log may not keep up. You could put the
commit log on the same RAID volume to see if that improves writes.

The config we tried modifying so far was concurrent_reads to (16 * number of drives) and concurrent_writes
to (8 * number of cores) as per
256 write threads is a lot. Make sure the commit log can keep up, I would put it back to 32,
maybe try 64. Not sure the concurrent list for the commit log will work well with that many
threads.

May want to put the reads down as well.

It's easier to tune the system if you can provide some info on the workload.

Cheers

-----------------
Aaron Morton
New Zealand
@aaronmorton

Co-Founder & Principal Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com

On 7/11/2013, at 12:35 pm, Arindam Barua <abarua@247-inc.com<mailto:abarua@247-inc.com>>
wrote:



We want to upgrade our Cassandra cluster to have newer hardware, and were wondering if anyone
has suggestions on Cassandra or linux config changes that will prove to be beneficial.
As of now, our performance tests (our application specific as well as cassandra-stress) are
not showing any significant difference in the hardwares, which is a little disheartening,
since the new hardware has a lot more RAM and CPU.

Old Hardware: 8 cores (2 quad core), 32 GB RAM, four 1-TB disks ( 1 disk used for commitlog
and 3 disks RAID 0 for data)
New Hardware: 32 cores (2 8-core with hyperthreading), 128 GB RAM, eight 1-TB disks ( 1 disk
used for commitlog and 7 disks RAID 0 for data)

Most of the cassandra config currently is the default, and we are using LeveledCompaction
strategy. Default key cache, row cache turned off.
The config we tried modifying so far was concurrent_reads to (16 * number of drives) and concurrent_writes
to (8 * number of cores) as per recommendation in cassandra.yaml, but that didn't make much
difference.
We were hoping that at least the extra RAM in the new hardware will be used for Linux file
caching and hence an improvement in performance will be observed.

Running Cassandra 1.1.5 currently, but evaluating to upgrade to 1.2.11 soon.

Thanks,
Arindam


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