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Re: The direction of malloc?
- From: Siddhesh Poyarekar <siddhesh at redhat dot com>
- To: Will Newton <will dot newton at linaro dot org>
- Cc: OndÅej BÃlka <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: Thu, 12 Dec 2013 09:18:28 +0530
- 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> <CANu=DmiTFu59qTgP=3Ks6_biCGvGqnis0++mmZdqX6-1FDWaUg at mail dot gmail dot com>
On Wed, Dec 11, 2013 at 09:15:01AM +0000, Will Newton wrote:
> On 11 December 2013 02:31, Siddhesh Poyarekar <firstname.lastname@example.org> 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.
I think I misread Ondrej's post and thought he meant 'lock contention'
when he actually only mentioned 'cost of atomic operations'. I agree
that we need a lockless fast path.