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Re: The direction of malloc?
- From: Will Newton <will dot newton at linaro dot org>
- To: Siddhesh Poyarekar <siddhesh at redhat dot com>
- Cc: Ondřej Bílka <neleai at seznam dot cz>, Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>, libc-alpha <libc-alpha at sourceware dot org>
- Date: Wed, 11 Dec 2013 09:15:01 +0000
- Subject: Re: The direction of malloc?
- Authentication-results: sourceware.org; auth=none
- References: <52A6A0DA dot 1080109 at redhat dot com> <CANu=Dmi32gwk-hQ3dDbj0d4_gs3FWqt02+NmveXH1p03Vm+Mfg at mail dot gmail dot com> <20131210121622 dot GA5416 at domone dot podge> <52A75502 dot 6040500 at linux dot vnet dot ibm dot com> <20131210210541 dot GA19161 at domone dot podge> <20131211023150 dot GA20835 at spoyarek dot pnq dot redhat dot com>
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