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Date: Wed, 21 Sep 2022 13:58:17 -0400
From: Rich Felker <dalias@...c.org>
To: 王志强 <00107082@....com>
Cc: musl@...ts.openwall.com, Quentin Rameau <quinq@...th.space>,
Florian Weimer <fweimer@...hat.com>
Subject: Re: Re:Re: The heap memory performance
(malloc/free/realloc) is significantly degraded in musl 1.2 (compared to
1.1)
On Wed, Sep 21, 2022 at 01:15:35PM -0400, Rich Felker wrote:
> On Wed, Sep 21, 2022 at 06:15:02PM +0800, 王志强 wrote:
> > Hi Rich,
> >
> >
> >
> > I am quite interested into the topic, and made a comparation between glibc and musl with following code:
> > #define MAXF 4096
> > void* tobefree[MAXF];
> > int main() {
> > long long i;
> > int v, k;
> > size_t s, c=0;
> > char *p;
> > for (i=0; i<100000000L; i++) {
> > v = rand();
> > s = ((v%256)+1)*1024;
> > p = (char*) malloc(s);
> > p[1023]=0;
> > if (c>=MAXF) {
> > k = v%c;
> > free(tobefree[k]);
> > tobefree[k]=tobefree[--c];
> > }
> > tobefree[c++]=p;
> > }
> > return 0;
> > }
> > ```
> >
> > The results show signaficant difference.
> > With glibc, (running within a debian docker image)
> > # gcc -o m.debian -O0 app_malloc.c
> >
> > # time ./m.debian
> > real 0m37.529s
> > user 0m36.677s
> > sys 0m0.771s
> >
> > With musl, (runnign within a alpine3.15 docker image)
> >
> > # gcc -o m.alpine -O0 app_malloc.c
> >
> > # time ./m.alpine
> > real 6m 30.51s
> > user 1m 36.67s
> > sys 4m 53.31s
> >
> >
> >
> > musl seems spend way too much time within kernel, while glibc hold most work within userspace.
> > I used perf_event_open to profile those programs:
> > musl profiling(total 302899 samples) shows that those "malloc/free" sequence spend lots of time dealing with pagefault/munmap/madvise/mmap
> >
> > munmap(30.858% 93469/302899)
> > _init?(22.583% 68404/302899)
> > aligned_alloc?(89.290% 61078/68404)
> > asm_exc_page_fault(45.961% 28072/61078)
> > main(9.001% 6157/68404)
> > asm_exc_page_fault(29.170% 1796/6157)
> > rand(1.266% 866/68404)
> > aligned_alloc?(20.437% 61904/302899)
> > asm_exc_page_fault(56.038% 34690/61904)
> > madvise(13.275% 40209/302899)
> > mmap64(11.125% 33698/302899)
> >
> >
> > But glibc profiling (total 29072 samples) is way much lighter, pagefault is the most cost while glibc spend significat time on "free"
> >
> >
> >
> > pthread_attr_setschedparam?(82.021% 23845/29072)
> > asm_exc_page_fault(1.657% 395/23845)
> > _dl_catch_error?(16.714% 4859/29072)__libc_start_main(100.000% 4859/4859)
> > cfree(58.839% 2859/4859)
> > main(31.138% 1513/4859)
> > asm_exc_page_fault(2.115% 32/1513)
> > pthread_attr_setschedparam?(3.725% 181/4859)
> > random(2.099% 102/4859)
> > random_r(1.832% 89/4859)
> > __libc_malloc(1.420% 69/4859)
> > It seems to be me, glibc make lots of uasage of cache of kernel
> > memory and avoid lots of pagefault and syscalls.
> > Is this performance difference should concern realworld
> > applications? On average, musl actual spend about 3~4ns per
> > malloc/free, which is quite acceptable in realworld applications, I
> > think.
> >
> >
> >
> > (Seems to me, that the performance difference has nothing to do with
> > malloc_usable_size, which may be indeed just a speculative guess
> > without any base)
>
> Indeed this has nothing to do with it. What you're seeing is just that
> musl/mallocng return freed memory, and glibc, basically, doesn't
> (modulo the special case of large contiguous free block at 'top' of
> heap). This inherently has a time cost.
>
> mallocng does make significant efforts to avoid hammering mmap/munmap
> under repeated malloc/free, at least in cases where it can reasonably
> be deemed to matter. However, this is best-effort, and always a
> tradeoff on (potential) large unwanted memory usage vs performance.
> More on this later.
>
> Your test case, with the completely random size distribution across
> various large sizes, is likely a worst case. The mean size you're
> allocating is 128k, which is the threshold for direct mmap/munmap of
> each allocation, so at least half of the allocations you're making can
> *never* be reused, and will always be immediately unmapped on free. It
> might be interesting to change the scaling factor from 1k to 256 bytes
> so that basically all of the allocation sizes are in the
> malloc-managed range.
One observation if this change is made: it looks like at least 70% of
the time is spent performing madvise(MADV_FREE), and that a large
portion of the rest (just looking at strace) seems to be repeatedly
mapping and freeing a 17-page (68k) block, probably because this size
happens to be at the boundary of some threshold where bounce
protection isn't happening. I think we should look at both of these in
more detail, since they both suggest opportunities for large
performance improvements at low cost.
Rich
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