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Date: Tue, 30 Oct 2018 11:00:39 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: musl: about malloc 'expand heap' issue
On Tue, Oct 30, 2018 at 11:11:07AM +0000, zhangwentao (M) wrote:
>
> Hi all,
> I am using musl in my project and I found an issue about the malloc function in musl:
>
> Issue Description:
> * When in muti-threads environment, malloc/free are called in high concurrency<http://dict.cn/high%20concurrency>.
>
> Malloc:
> Will find 'struct bin' from bitmap(without lock), and allocate memory from the bin (with lock).
>
> Free:
> Will merge the chunk together if the free memory is 'connected' to the existing chunk.
>
> ? It will remove the old chunk first then combine the chunk to a larger one..
>
> ? After merge operation done, insert the chunk to the bin list.
>
> ? Each of the chunk operation is locked while merging, but the whole steps aren't within a lock.
>
> So here is the issue:
>
> 1. There is only one chunk in largest bin list, and Free is on process, just remove the largest bins chunk from bin, the bitmap(mal.binmap) on that bit will be zero.
>
> 2. A malloc comes, the bitmap is zero, and goes to expand heap. (Actually there is enough memories in process)
>
> 3. Free operation goes on, and put the merged big chunk to bins.
>
> But in operation 2, the process has expand heap.
>
> If we have a loop on step 1-3, the process will expand heap frequently.
> So it will cost more Virtual Memory (of course, physical memory would be freed by calling '__madvise' if the chunk is big enough)
>
> In my environment , we do not have that much virtual memory. I think stop expand heap would a better choice.
>
> Do you have plan to fix it ??
This is a known issue, and intended to be fixed in the complete
redesign of malloc. Fixing it right in the current design seems to
impose significant performance costs that I thought were equivalent
to, or worse than, just using one global lock. However if it's causing
major problems I may be able to make a quick fix that's not too
expensive -- I'll take a look again today or tomorrow.
Rich
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