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Average cost of the OCaml GC
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Date: -- (:)
From: Michael Ekstrand <michael@e...>
Subject: Re: [Caml-list] Average cost of the OCaml GC
On 11/11/2010 07:52 AM, Jianzhou Zhao wrote:
> On Thu, Nov 11, 2010 at 4:08 AM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>> Jianzhou Zhao <jianzhou@seas.upenn.edu> writes:
>>
>>> Hi,
>>>
>>> What is the average cost of the OCaml GC? I have a program that calls
>>> 'mark_slice' in 57% of the total execution time, and calls
>>> 'sweep_slice' in 21% of the total time, reported by Callgrind, which
>>> is a profiling tool in Valgrind. 57% and 21% are the 'self cost' ---
>>> the cost of the function itself ('Self Cost'), rather than the cost
>>> including all called functions ('Inclusive Cost'). I guess
>>> 'mark_slice'  and  'sweep_slice'  are functions from OCaml GC. Are
>>> these numbers normal?
>>
>> Those numbers sound rather high to me.

They sound high to me as well, but not unheard of - I sometimes measure
a lot of time in the GC.

>>> My program calls both OCaml and C, which passes around C data types in
>>> between. I also doubt if I defined the interface in an 'unefficient'
>>> way that slows down the GC. Are there any rules in mind to make GC
>>> work more efficiently?
>>
>> You can tune some of the GC parameters to suit your use case.
>>
>> Do you allocate custom types from C? In caml_alloc_custom(ops, size,
>> used, max) the used and max do influence the GC how often to run.
> 
> Yes. The code uses caml_alloc_custom to create a lot of small objects
> (less then 8 bytes) frequently. The used and max are set to be
> default, 0 and 1. The manual says
>   http://caml.inria.fr/pub/docs/manual-ocaml/manual032.html#toc140
> 
> /////////////////////
> If your finalized blocks contain no pointers to out-of-heap resources,
> or if the previous discussion made little sense to you, just take used
> = 0 and max = 1. But if you later find that the finalization functions
> are not called “often enough”, consider increasing the used / max
> ratio.
> //////////////////////
> 
> Does this mean the default used and max let GC do finalization 'as
> slow as possible'? This does not seem to be the case if the costs 57%
> and 20% are too high.

Yes, with respect to GC cycles triggered by "too much" custom data
allocation.

There are a variety of things that can cause GC thrashing.  One of them
is the GC "space overhead" parameter, which controls how aggressive the
GC is at reclaiming memory.  Another is your minor heap size - if your
minor heap is too small, it can cause excess GC activity.  I documented
the parameter tuning I have done to reduce GC cost on my blog[1], but
here's a short summary:

 * Increase minor heap size.  I usually use 1M or 4M words; my general
rule of thumb is that I want one "work unit" with its temporary storage
requirements to fit in a minor heap.  This decreases the frequency both
of minor collections and major slices.
 * Increase space_overhead; I increase this to 100 or 200 (the default
is 80), as I typically run my large codes on machines with lots of spare
RAM and can accept a space-speed tradeoff.
 * Increase the heap increment.  If your process will require lots of
RAM, this lets it allocate that memory in bigger chunks further
decreasing the memory overhead.

I also use a patched Bigarray that allows me to set the "max" parameter
it uses in its invocations of caml_alloc_custom, but if you are not
using bigarray that shouldn't be impacting your program's performance.
It's quite critical when allocating large bigarrays, though!  Having
custom blocks allocated near or above the "max" param is a sure-fire
recipe for GC thrashing.  It sounds like you're avoiding that pitfall,
though.

So, the short short story: you're doing many of the right things
(measuring, not letting custom allocations thrash the GC).  Some more
parameter tuning will hopefully help you decrease your GC overhead.

1. http://elehack.net/michael/blog/2010/06/ocaml-memory-tuning

- Michael