Re: the source of slow large for loops

[Mikael Magnusson <mikachu@gmail.com>]

On 7 May 2011 21:11, Bart Schaefer <schaefer@brasslantern.com> wrote:
> On May 7,  7:00pm, Mikael Magnusson wrote:
> } Subject: the source of slow large for loops
> }
> } I don't know if anyone looked into why things like
> } for i in {1..700000}; do true; done
> } is extremely slow in zsh, so I did now. Turns out zhalloc is extremely
> } slow
>
> Hmm ... I think you've misdiagnosed.  zhalloc() is reasonably fast.
> What's amazingly slow is freeheap().  If I stick a "return;" at the
> top of freeheap() the above loop runs in just over 4 seconds on my
> 3GHz P4.  (AFAICT the memory is still eventually freed by popheap(),
> it just grows a lot more before releasing any.)

Ah, and when I made zhalloc a nop, freeheap() is fast again because
there are no heaps.

> } and for the above loop, one particular line runs 12049901132
> } times according to gcov.
>
> It would have been nice if you'd told us which line. :-)  However, I
> don't think that's actually the problem.  There are a couple of places
> where zhalloc() and friends search the heap for free space by doing a
> linear scan over a linked list, which will cause a huge number of loop
> iterations, but each of those iterations is at most a couple of machine
> instructions.  You need to look at how much time something takes, not
> just how often it's done.

Yeah, I actually tried gprof first, but I was unable to start zsh
then, it just exited with "profile signal" or something like that.

> The real time sink is this bit of freeheap():
>
>    for (h = heaps; h; h = hn) {
>        hn = h->next;
>        if (h->sp) {
> #ifdef ZSH_MEM_DEBUG
>            memset(arena(h) + h->sp->used, 0xff, h->used - h->sp->used);
> #endif
>            h->used = h->sp->used;
>            if (!fheap && h->used < ARENA_SIZEOF(h))
>                fheap = h;
>            hl = h;
>        } else {
> #ifdef USE_MMAP
>            munmap((void *) h, h->size);
> #else
>            zfree(h, HEAPSIZE);
> #endif
>        }
>    }
>
> Of course if you've used --enable-zsh-mem-debug for configure then this
> is also zeroing all the memory on each free and the performance is even
> worse, but that isn't really the issue.
>
> Most specifically, take out the stuff inside "if (h->sp)" and the above
> 700k-iterations loop runs in 11 seconds.
>
> If I understand what's going on here ...
>
> Index: Src/mem.c
> ===================================================================
> RCS file: /extra/cvsroot/zsh/zsh-4.0/Src/mem.c,v
> retrieving revision 1.10
> diff -c -r1.10 mem.c
> --- Src/mem.c   4 Nov 2008 04:47:53 -0000       1.10
> +++ Src/mem.c   7 May 2011 19:06:23 -0000
> @@ -220,8 +220,28 @@
>     h_free++;
>  #endif
>
> +    /* At this point we used to do:
>     fheap = NULL;
> -    for (h = heaps; h; h = hn) {
> +     *
> +     * When pushheap() is called, it sweeps over the entire heaps list of
> +     * arenas and marks every one of them with the amount of free space in
> +     * that arena at that moment.  zhalloc() is then allowed to grab bits
> +     * out of any of those arenas that have free space.
> +     *
> +     * With the above reset of fheap, the loop below sweeps back over the
> +     * entire heap list again, resetting the free space in every arena to
> +     * the amount stashed by pushheap() and finding the first arena with
> +     * free space to optimize zhalloc()'s next search.  When there's a lot
> +     * of stuff already on the heap, this is an enormous amount of work,
> +     * and peformance goes to hell.
> +     *
> +     * However, there doesn't seem to be any reason to reset fheap before
> +     * beginning this loop.  Either it's already correct, or it has never
> +     * been set and this loop will do it, or it'll be reset from scratch
> +     * on the next popheap().  So all that's needed here is to pick up
> +     * the scan wherever the last pass [or the last popheap()] left off.
> +     */
> +    for (h = (fheap ? fheap : heaps); h; h = hn) {
>        hn = h->next;
>        if (h->sp) {
>  #ifdef ZSH_MEM_DEBUG
> @@ -242,7 +262,7 @@
>     if (hl)
>        hl->next = NULL;
>     else
> -       heaps = NULL;
> +       heaps = fheap = NULL;
>
>     unqueue_signals();
>  }

-- 
Mikael Magnusson