Removing latency spikes. Garbage collector related?

Will Sewell me at
Tue Sep 29 09:35:55 UTC 2015

Thank you for the reply Neil.

The spikes are in response time. The graph I linked to shows the
distribution of response times in a given window of time (darkness of
the square is the number of messages in a particular window of
response time). So the spikes are in the mean and also the max
response time. Having said that I'm not exactly sure what you mean by
"mean values".

I will have a look into -I0.

Yes the arrival of messages is constant. This graph shows the number
of messages that have been published to the system:

On 29 September 2015 at 10:16, Neil Davies
<semanticphilosopher at> wrote:
> Will
> is your issue with the spikes i response time, rather than the mean values?
> If so, once you’ve reduced the amount of unnecessary mutation, you might want
> to take more control over when the GC is taking place. You might want to disable
> GC on timer (-I0) and force GC to occur at points you select - we found this useful.
> Lastly, is the arrival pattern (and distribution pattern) of messages constant or
> variable? just making sure that you are not trying to fight basic queueing theory here.
> Neil
> On 29 Sep 2015, at 10:03, Will Sewell <me at> wrote:
>> Thanks for the reply Greg. I have already tried tweaking these values
>> a bit, and this is what I found:
>> * I first tried -A256k because the L2 cache is that size (Simon Marlow
>> mentioned this can lead to good performance
>> * I then tried a value of -A2048k because he also said "using a very
>> large young generation size might outweigh the cache benefits". I
>> don't exactly know what he meant by "a very large young generation
>> size", so I guessed at this value. Is it in the right ballpark?
>> * With -H, I tried values of -H8m, -H32m, -H128m, -H512m, -H1024m
>> But all lead to worse performance over the defaults (and -H didn't
>> really have much affect at all).
>> I will try your suggestion of setting -A to the L3 cache size.
>> Are there any other values I should try setting these at?
>> As for your final point, I have run space profiling, and it looks like
>>> 90% of the memory is used for our message index, which is a temporary
>> store of messages that have gone through the system. These messages
>> are stored in aligned chunks in memory that are merged together. I
>> initially though this was causing the spikes, but they were still
>> there even after I removed the component. I will try and run space
>> profiling in the build with the message index.
>> Thanks again.
>> On 28 September 2015 at 19:02, Gregory Collins <greg at> wrote:
>>> On Mon, Sep 28, 2015 at 9:08 AM, Will Sewell <me at> wrote:
>>>> If it is the GC, then is there anything that can be done about it?
>>> Increase value of -A (the default is too small) -- best value for this is L3
>>> cache size of the chip
>>> Increase value of -H (total heap size) -- this will use more ram but you'll
>>> run GC less often
>>> This will sound flip, but: generate less garbage. Frequency of GC runs is
>>> proportional to the amount of garbage being produced, so if you can lower
>>> mutator allocation rate then you will also increase net productivity.
>>> Built-up thunks can transparently hide a lot of allocation so fire up the
>>> profiler and tighten those up (there's an 80-20 rule here). Reuse output
>>> buffers if you aren't already, etc.
>>> G
>>> --
>>> Gregory Collins <greg at>
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