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Re: Firefox still crashes

From: Tomas Bodzar <tomas.bodzar@xxxxxxxxx>
Date: Fri, 15 Oct 2010 06:52:25 +0200

That's an excellent reading. It will be great to read more about
technologies in DragonflyBSD. Something like
http://www.openbsd.org/papers ? ;-)

On Fri, Oct 15, 2010 at 3:44 AM, Venkatesh Srinivas
<me@endeavour.zapto.org> wrote:
>>> What's a "magazine"?
>>> Pierre
> In libc, nmalloc (lib/libc/stdlib/nmalloc.c) provides malloc() ( from malus
> locus, 'bad place' ) and free() for single-threaded and multithreaded
> applications. In the DragonFly 2.4 release cycle, the original allocator
> (phkmalloc, inherited from FreeBSD) was replaced with a port of the kernel
> slab allocator; in the 2.8 release cycle I committed some work to change the
> multithreaded strategy.
> The DragonFly 2.4 and 2.6 libc allocator had two strategies, one for small
> requests, one for large requests; large requests are served directly via
> mmap. Small requests are served from 64k, 64k-aligned regions of memory
> called 'slabs'. Each slab only services requests of a given size, minimizing
> fragmentation. (The DragonFly libc slab allocator was fairly different from
> the original Sun design -- the Sun design has variable-size slabs and a hash
> table for block->slab mappings). For multithreaded applications, the
> allocator kept track of four sets of slab structures; threads would attempt
> to use the set they'd used most recently. If they failed to lock that said,
> they'd move on to the next set.
> The 2.7/2.8 allocator has a new structure -- a magazine. A magazine is a
> fixed-size array of blocks of the same size. Each thread carries a pair of
> magazines; when a thread tries to allocate something, it first checks its
> magazines for a buffer _without any locking_. If the magazines are not able
> to support an allocation, a central collection of magazines, called the
> 'depot', is locked and a magazine is retrieved. If the depot is empty, we
> fall back to the slab allocator. The design is also from Sun -- see a paper
> called 'Magazines and Vmem: Extending the Slab Allocator to Many CPUs and
> Multiple Resources'. When I last measured, in the 2.6 release cycle, the
> magazine layer sped up sysbench OLTP / MySQL by approximately 20%.
> The 2.7/2.8 allocator also has work to reduce the number of mmap/munmap
> system calls relative to the earlier version of the allocator; rather than
> immediately unmapping a slab when it has no outstanding allocations, we keep
> around up to 64 old slabs and we attempt to allocate slabs in bursts from
> the system. When I last measured, the reduction in mmaps/munmaps was fairly
> dramatic.
> The latest bug was a fairly unfortunate one -- most of the locks in nmalloc
> use libc's spinlocks. The depot locks, however, used pthread_spinlocks; when
> nmalloc was linked against libc, it used the stub pthread_spinlocks in libc,
> rather than the versions in libthread. This meant that accesses to the depot
> magazine lists were not synchronized at all and the magazine lists were
> getting corrupted. Oops...
> -- vs

“If you’re good at something, never do it for free.” —The Joker

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