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From James Golick <jamesgol...@gmail.com>
Subject Re: Row slice / cache performance
Date Sun, 02 May 2010 18:01:14 GMT
Not sure why the first paragraph turned in to a numbered bullet...

On Sun, May 2, 2010 at 11:00 AM, James Golick <jamesgolick@gmail.com> wrote:

>
>    1. I wrote the list a while back about less-than-great performance when
>    reading thousands of columns even on cache hits. Last night, I decided to
>    try to get to the bottom of why.
>
>
> I tested this by setting the row cache capacity on a TimeUUIDType-sorted CF
> to 10, filling up a single row with 2000 columns, and only running queries
> against that row. That row was the only thing in the database. I rm -Rf'd
> the data before starting the test.
>
> The tests were done from Coda Hale's scala client cassie, which is just a
> thin layer around the java thrift bindings. I didn't actually time each call
> because that wasn't the objective, but I didn't really need to. Reads of 10
> columns felt quick enough, but 100 columns was slower. 1000 columns would
> frequently cause the client to timeout. The cache hit rate on that CF was
> 1.0, so, yes, the row was in cache.
>
> Doing a thousand reads with count=100 in a single thread pegged my
> macbook's CPU and caused the fans to spin up pretty loud.
>
> So, I attached a profiler and repeated the test. I'm no expert on cassandra
> internals, so please let me know if I'm way off here. The profiled reads
> were reversed=true, count=100.
>
> As far as I can tell, there are three components taking up most of the time
> on this type of read (row slice out of cache):
>
>    1. ColumnFamilystore.removeDeleted() @ ~40% - Most of the time in here
>    is actually spent materializing UUID objects so that they can be compared in
>    the ConcurrentSkipListMap (ColumnFamily.columns_).
>    2. SliceQueryFilter.getMemColumnIterator @ ~30% - Virtually all the
>    time in here is spent in ConcurrentSkipListMap$Values.toArrray()
>    3. QueryFilter.collectCollatedColumns @ ~30% - All the time being spent
>    in ColumnFamily.addColumn, and about half of the total spent materializing
>    UUIDs for comparison.
>
> This profile is consistent with the decrease in performance with higher
> values for count. If there are more UUIDs to deserialize, the performance of
> removeDeleted(), and collectCollatedColumns() should increase (roughly)
> linearly.
>
> So, my question at this point is how to fix it. I have some basic ideas,
> but being new to cassandra internals, I'm not sure they make any sense. Help
> me out here:
>
>    1. Optionally call removeDeleted() less often. I realize that this is
>    probably a bad idea for a lot of reasons, but it was the first thing I
>    thought of.
>    2. When a ColumnFamily object is put in to the row cache, copy the
>    columns over to another data structure that doesn't need to be sorted on
>    get(). If columns_ needs to be kept around, this option would have a memory
>    impact, but at least for us, it'd be well worth it for the speed.
>    3. ????
>
> I'd love to hear feedback on these / the rest of this (long) post.
>

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