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From Keith Freeman <8fo...@gmail.com>
Subject Re: heavy insert load overloads CPUs, with MutationStage pending
Date Tue, 10 Sep 2013 17:40:23 GMT

On 09/10/2013 11:17 AM, Robert Coli wrote:
> On Tue, Sep 10, 2013 at 7:55 AM, Keith Freeman <8forty@gmail.com 
> <mailto:8forty@gmail.com>> wrote:
>
>     On my 3-node cluster (v1.2.8) with 4-cores each and SSDs for
>     commitlog and data
>
>
> On SSD, you don't need to separate commitlog and data. You only win 
> from this separation if you have a head to not-move between appends to 
> the commit log. You will get better IO from a strip with an additional 
> SSD.
Right, actually both partitions are on the same SSD.   Assuming you 
meant "stripe", would that really make a difference
>
>         Pool Name                    Active   Pending  Completed  
>         Blocked  All time blocked
>         MutationStage                     1         9 290394         0
>                         0
>         FlushWriter                       1         2   20         0  
>                       0
>
>     I can't seem find information about the real meaning of
>     MutationStage, is this just normal for lots of inserts?
>
>
> The mutation stage is the stage in which mutations to rows in 
> memtables ("writes") occur.
>
> The FlushWriter stage is the stage that turns memtables into SSTables 
> by flushing them.
>
> However, 9 pending mutations is a very small number. For reference on 
> an overloaded cluster which was being written to death I recently 
> saw.... 1216434 pending MutationStage. What problem other than "high 
> CPU load" are you experiencing? 2 Pending FlushWriters is slightly 
> suggestive of some sort of bound related to flushing..
So the basic problem is that write performance is lower than I 
expected.  I can't get sustained writing of 5000 ~1024-byte records / 
sec at RF=2 on a good 3-node cluster, and my only guess is that's 
because of the heavy CPU loads on the server (loads over 10 on 4-CPU 
systems).  I've tried both a single client writing 5000 rows/second and 
2 clients (on separate boxes) writing 2500 rows/second, and in both 
cases the server(s) doesn't respond quickly enough to maintain that 
rate.  It keeps up ok with 2000 or 3000 rows per second (and has lower 
server loads).



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