Thoughts about malloc(3) and threading.

[Poul-Henning Kamp]

A number of times the issue of memory allocation in threaded programs
have come up in various fora.

In order to not become a road block for this important area, I want
to state my opinion on this matter for the record:

For single-threaded programs and non-malloc intensive multi-threaded
programs I am still of the opinion that phkmalloc is our best choice.

The combination of strong error checking and focus on minimal working
set size seems to me a good compromise between functionality and
raw performance for a general purpose malloc in a general purpose
operating system.

For multi-threaded programs with a lot of malloc activity, phkmalloc
does not work well since it requires one big lock around the entire
thing and does not do anything to preserve processor affinity.

I think we need a malloc which can service this hi-end segment
effectively, but even though various suggestions for replacements
of phkmalloc to address this have been made, I do not see any clear
winner or even the full list of candidates yet.

Since a malloc which is aware of multiple CPUs or which relies on
per-cpu locks can not implement a strong level of error checking,
my guess is that we will end up retaining phkmalloc as the default
and have a "SMP malloc" for the heavy duty case.

It can (and without doubt will) be argued that the "SMP malloc"
should be a port, but lets not even go there until we have a
candidate.

So, short version of my position is:

	I am not and will not be researching this topic and that
	people with a serious interest in it should get started.

	Given the list of programs which have been found buggy by
	phkmalloc, I do not think we should default to a malloc which
	does not have strong error checking.

	If a malloc better than phkmalloc in performance and error
	checking is found/written, I'll be all for a replacement
	if that means we will only have one malloc.

Anyone wanting to work on this is welcome to contact me for
some hard-earned insight into what and how to benchmark etc.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
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FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.