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Date: Mon, 3 Aug 2020 00:22:14 +0800
From: Zhao Zhengyu <zhaozhengyu0@...il.com>
To: musl@...ts.openwall.com
Subject: Re: When to reclaim pages in __bin_chunk?
I read the code of mallocng carefully and found some ideas similar to my
current work: For malloc requirements of the “small size”, I split one page
into the specified size, and manage them as a group. When free, I give
them back to heap together. But at the point of reclaiming physical pages
to kernel, I can't get enough inspiration from mallocng.
Anyway, I understand the question I asked. Thanks!
On Sat, Aug 1, 2020 at 3:24 AM Rich Felker <dalias@...c.org> wrote:
> On Fri, Jul 31, 2020 at 07:22:16PM +0200, Markus Wichmann wrote:
> > On Fri, Jul 31, 2020 at 08:17:02AM +0800, Zhao Zhengyu wrote:
> > > Hello,
> > >
> > > When chunks are merged, we use "(curr_size + pre_size) ^ pre_size >
> > > pre_size" to decide whether to reclaim. I think this may be something
> > > related to performance, but I can’t prove it. I want to know the
> > > reason.
> > >
> > > Thank you!
> > >
> > > Zhengyu
> >
> > I asked that same question a while ago. For one, this was in the old
> > malloc code, which is now usually no longer used. For two, this tries to
> > figure out if adding current size to previous size rolls over into a new
> > power of two. Usually, curr_size will be small and pre_size will be
> > large. Therefore, adding the two will not change much about the high
> > bits of pre_size, so due to the XOR operator, those bits will cancel out
> > and the result will be smaller than pre_size. However, if the sum does
> > roll over, then the sum has one bit set that isn't in pre_size and is
> > larger than all the ones in pre_size. So the XOR can't cancel that bit,
> > and the result of the XOR is greater than pre_size.
> >
> > Fundamentally, it is an optimized version of (a_clz(curr_size +
> > pre_size) < a_clz(pre_size)).
>
> Yes, it's a heuristic at least approximately equivalent to "crossed
> the next power of two size boundary" to limit the frequency of madvise
> syscalls when a large free zone keeps getting expanded by adjacent
> tiny frees. However it does not work very well in practice, and
> doesn't even mitigate the possibility of continuous syscall load when
> a repeated malloc/free cycle occurs right at a power-of-two boundary.
>
> mallocng handles this kind of thing much better by grouping same-sized
> allocations and returning them as a group when all are freed, only
> holding back from doing so if it's observed allocations of this size
> "bouncing" (repeatedly creating and destroying the same group).
>
> Rich
>
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