Re: [Caml-list] Slow GC problem

[Shivkumar Chandrasekaran <shiv@ece.ucsb.edu>]

Pursuing my earlier enquiry: I noticed that module Bigarray allocated 
its arrays using "malloc" rather than on the ocaml heap. My problem 
allocates a lot (say 100,000) bigarrays of rather small size (say 30 x 
30). Can this potentially increase the cost of gc? (I have this mental 
picture of the ocaml heap fragmented by these immovable malloc'd 
bigarrays.) Any help will be appreciated. Thanks,

--shiv--


On Friday, April 4, 2003, at 11:40 AM, Shivkumar Chandrasekaran wrote:

> I have a gc efficiency problem for which I require some advice. I have 
> read both the O'Reilly book and the manual on gc.
>
> I am implementing a fast direct matrix solver for 2D PDEs. So it uses 
> the Bigarray module a lot. I have two versions of my algorithm. On is 
> an in-core algorithm and the other is the same solver, except that it 
> is out-of-core (most of the matrices are stored in disk files). 
> Unfortunately the out-of-core solver  is *faster* than the in-core 
> solver for the identical problem! I was expecting the out-of-core 
> solver to be 10 times slower. I am concluding that gc is to blame. 
> Below I give the gc stats just before and after the solver routine is 
> called in the in-core solver:
>
> 				"Just before"	"Just after"
> minor_words:		46243376	139259767
> promoted_words:	928267		2595523
> major_words:		2883087		39489766
> minor_collections:	1412		4591
> major_collections:	18			52
> heap_words:		2150400		1044480
> heap_chunks:		35			17
> top_heap_words:	2150400		5038080
> live_words:		1842373		840037
> live_blocks:		253926		116816
> free_words:		307180		204440
> free_blocks:		47368		17
> largest_free:		10928		61440
> fragments:		847			3
> compactions:		0			2
>
> I tried changing some parameters using Gc.set but it did not make a 
> significant difference. Does anybody see any obvious gc problems from 
> the above data? Thanks,
>
> --shiv--
>
>
> PS: I wrote the out-of-core solver in just 3 days once the in-core 
> solver was done, all in O'Caml. This would have have taken much longer 
> in Fortran/C. Thanks to the O'Caml team.
>
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--shiv--

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