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Re: The direction of malloc?


On 11 December 2013 02:31, Siddhesh Poyarekar <siddhesh@redhat.com> wrote:
> On Tue, Dec 10, 2013 at 10:05:41PM +0100, OndÅej BÃlka wrote:
>> > * Should we provide thread cache blocks to do provide some lockless allocation?
>>
>> This is most low-hanging fruit that I aim for. We already use tls to
>> determine arena so this should not be a issue.
>>
>> We have fastbins that sorta do this but with several problems.
>> 1. They are not really lockless, for malloc they need a lock, only
>> freeing will be when bug 15073 gets fixed.
>>
>> Second problem is that fastbins are per-arena not per-thread which
>> forces us to use atomic operations. These are expensive (typicaly more than 50 cycles).
>>
>> Moving these to per-thread bins mostly just needs refactoring of current
>> code to one that makes more sense.
>
> With arenas-per-thread, you essentially have contention-free access,
> which is not the same thing as lock-free, but not much worse.  You'll
> have lock contention in per-thread arenas only when there are more
> threads than arenas, which in the default case means that you have
> more threads than twice the number of cores, which is too many threads
> anyway.

Lock contention would be worse, but still the atomic instructions
required to lock/unlock the arena is the hottest part of the profile
on many single-threaded malloc workloads.

If we are going to get a new malloc or update the old one I think the
fast path being lock-free should be a requirement.

-- 
Will Newton
Toolchain Working Group, Linaro


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