[Haskell-cafe] Where did these threads come from?

David Turner dct25-561bs at mythic-beasts.com
Fri Nov 13 11:50:21 UTC 2015

Ah yes, and the more obvious question: should I just run it with -I0? I'm
kind of assuming that the idle GC is a Good Thing since it's on by default.


On 13 November 2015 at 11:11, David Turner <dct25-561bs at mythic-beasts.com>

> Hi all,
> I'm working on a server process that has (by design) fairly high memory
> usage but serves relatively few requests and spends most of its time idle.
> By 'fairly high' I mean 100s of MB, nothing ridiculous. I'm trying to
> improve its quiescent CPU usage as this is currently the limiting factor
> for how cheaply we can run this service - we're aiming for the cheapest
> t-class EC2 machine we can get.
> The quiescent CPU usage is currently about 50%. This seems to be largely
> due to the idle GC, because running with -I0 cuts it down to about 0.3%.
> Pretty much every time any thread wakes up it triggers the idle GC a short
> while later and because of the high memory usage these GCs are somewhat
> expensive.
> Using threadscope I found the biggest problem was the reaper thread in
> resource-pool which wakes up every second whether there's anything to reap
> or not. Fortunately this was easy to spot as the reaper thread labels
> itself as such. I've replaced this with an event-driven reaper that only
> wakes up on demand and cut the baseline from 50% down to just under 10%.
> There are two more threads I can now see waking up periodically in
> threadscope, with respective periods approximately 10 sec and 30 sec. I
> have a hunch what the 30-sec one is but no idea about the 10-sec one. If I
> can kill off the 10 sec one then that should take CPU usage down to <5%
> which would be just fine.
> My question: is there any reliable way to determine where these threads
> came from, in terms of the location of the forkIO call that made them? I've
> just been looking for likely candidates and labelling threads and this is
> rather tedious.
> Cheers,
> David
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