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Date: Thu, 3 Apr 2014 20:17:25 +0200
From: "M. Ziebell" <ziebell_marco@...teo.de>
To: musl@...ts.openwall.com
Subject: Re: malloc & uncommitting memory
Am Thu, 3 Apr 2014 00:43:23 -0400
schrieb Rich Felker <dalias@...ifal.cx>:
> On Wed, Apr 02, 2014 at 05:08:05PM -0400, Rich Felker wrote:
> > As long as thresholds are chosen right, I don't think this approach
> > would be costly from a performance standpoint. But it might have
> > consequences for fragmentation. So I'd like to discuss it further,
> > see what ideas emerge, and whether it looks reasonable before
> > trying to implement anything.
>
> Basically, the way fragmentation can result from having "frozen"
> chunks is when small chunks adjacent to them are freed: these free
> chunks are unable to be merged with the frozen chunk, so they get
> reused for small allocations despite being adjacent to a large free
> (albeit frozen) region.
>
> I see several possible ways to reduce or avoid this:
>
> 1. Always leave large padding (large enough to stay in bin-63) on both
> sides of the frozen chunk when freezing. This would force adjacent
> freed chunks to get merged into a bin-63 chunk, preventing them
> from getting used for small allocations unless nothing smaller were
> available. However, they could still get used and gradually
> depleted.
>
> 2. When a chunk is freed, check if it's a neighbor to a frozen chunk,
> and if so, put it at the end of its bin rather than the beginning,
> so that other free chunks of the appropriate size get used first.
> However this wouldn't help when it's the only chunk of its size but
> another slightly-larger chunk would be a better candidate to split.
>
> 3. Allow merging directly into frozen chunks, the same way merging
> into free chunks is done now, with logic to expand the region
> that's PROT_NONE similar to the current logic for expanding the
> MADV_DONTNEED region.
>
> Of these, option 3 looks the most promising.
>
> Rich
As far as i understand "uncommitted" memory is allocated address space.
A region in memory which is not used by the program, but the address
space expanded to make a smaller region available to the programm for
use.
Please correct me if I am wrong.
Based on that understanding, the idea or your proposal sounds not
really desirable.
You have to take care of tons of exceptions and special cases.
I'm pretty sure I don't understand what uncommitted memory is for, but
I'm heary is hundreds of lines of complex code, if-else constructs
everywhere and bugs.
The way you suggest to manage these regions in memory sound pretty and
advanced, please don't feel offended by my statement.
Marco
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