0004773: Array.make uses twice the required memory - MantisBT

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ID

Project

Category

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Date Submitted

Last Update

0004773

OCaml

OCaml general

public

2009-04-19 16:53

2012-07-11 14:54

Reporter

Camarade_Tux

Assigned To

Priority

normal

Severity

minor

Reproducibility

always

Status

resolved

Resolution

no change required

Platform

x86_64

OS

Linux x86_64

OS Version

Product Version

3.11.0

Target Version

Fixed in Version

Summary

0004773: Array.make uses twice the required memory

Description

Currently calling 'Array.make (16*1024*1024) 0' uses approximately twice the required memory. This doesn't show up immediately though : on 64bit, resident size climbs to (a bit more than) 128MB while virtual size goes up to (roughly) 230MB.
With linux's overcommit this isn't a problem at first but when compacting, the resident size increases to match the virtual one and I run out of memory.

I tried to pinpoint the problem myself but couldn't manage.

Is it possible to reduce the memory usage ?

Steps To Reproduce

let a = Array.make (16*1024*1024) 0 in
let () = Gc.compact () in
read_line ()

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Notes
(0004922) Camarade_Tux (reporter) 2009-04-19 23:24	I had forgotten about Bigarrays which have let me reduce memory usage (has been divided by three). Now, should int arrays take less memory or should the documentation be extended to warn about memory usage and point to bigarrays ?

(0004972) doligez (manager) 2009-05-26 17:30	This is (more or less) done on purpose. Because of the incremental garbage collector, for every 10 bytes that are in use by your data, there are another 8 bytes allocated to hold the dead data that will be reclaimed by the current (or the next) GC cycle. This 80% overhead is the "space_overhead" parameter of the GC. By lowering this parameter, you can reduce memory consumption at the cost of more time spent in the GC. In your case, if you have a few very large arrays with a very long lifetime, you should be able to reduce the space overhead quite a lot without paying too much in terms of run time. Unfortunately, I don't know of a simple way to do this automatically.

Issue History
Date Modified	Username	Field	Change
2009-04-19 16:53	Camarade_Tux	New Issue
2009-04-19 23:24	Camarade_Tux	Note Added: 0004922
2009-04-29 15:59	doligez	Status	new => acknowledged
2009-05-26 17:30	doligez	Note Added: 0004972
2009-05-26 17:31	doligez	Status	acknowledged => confirmed
2012-07-11 14:54	doligez	Status	confirmed => resolved
2012-07-11 14:54	doligez	Resolution	open => no change required

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