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Message-ID: <20181030150039.GT5150@brightrain.aerifal.cx> 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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