Re: perl malloc slow?

On Wed, 7 Jan 2004 11:49:16 +1100
Peter Jeremy <peterjeremy_at_optushome.com.au> wrote:

> >You need to make a context switch for every malloc call.
> 
> This isn't true.  There are no circumstances under which phkmalloc
> requires a context switch.  Since Unix uses a pre-emptive scheduler,
> a context switch may occur at any time, though it is preferentially
> performed during system calls.

Yes, wrong wording by me. I was trying to say it has to enter the kernel
via a syscall. In the worst case for every malloc()/free(). And entering
the kernel is expensive, compared to not entering the kernel and doing
stuff in userland.

> If the free memory pool managed by phkmalloc has insufficient space to
> fulfil the request, or is excessively large following a free() then it
> will use brk(2) to allocate/return additional memory.  The kernel may
> choose to schedule an alternative process during the brk() call.

With a sufficiently large amount of syscalls the wall clock time will
increase. And that's what Holger reported (further reading your mail:
not because of this fact, but I hadn't digged into the perl module).

[reason why the perl module behaves poorly]
> It's not clear why the builtin perl malloc is so much faster in this
> case.  A quick check of the perl malloc code suggests that it uses a
> geometric-progression bucket arrangement (whereas phkmalloc appears to
> use page-sized buckets for large allocations) - this would
> significantly reduce the number of realloc() copies.

This is IMHO the right allocation algorithm for such programs (at least
I don't know of a better one and I've seen it in several places where
you can't guess the amount of memory you need). I'm sure the perl
developers tuned the perl_malloc() with real world perl programs. Maybe
this kind of behavior is typical for a lot of perl programs.

Bye,
Alexander.

-- 
           I will be available to get hired in April 2004.

http://www.Leidinger.net                       Alexander _at_ Leidinger.net
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