Re: [PATCH] Remove atomic operations from malloc.c

[Rich Felker <dalias at libc dot org>]

On Wed, Feb 18, 2015 at 06:30:10PM +0530, Siddhesh Poyarekar wrote:
> On Wed, Feb 11, 2015 at 12:01:08PM +0100, Torvald Riegel wrote:
> > If your machine has just two cores, then at the very least you should
> > measure for just two threads too; a bigger number of threads is not
> > putting more contention on any of the synchronization bits, there's just
> > some more likelihood to having to wait for a thread that isn't running.
> > 
> > Also, to really assess performance, this has to be benchmarked on a
> > machine with more cores.  Additionally, you could argue why it should
> > not make a difference, and if that's a compelling argument, we could
> > follow it instead of the benchmark (which, as Will mentions, is hard to
> > make representative of real-world workloads).
> 
> The default malloc implementation creates 8 * n arenas on a system
> with n cores, so for anything up to 8 * n threads, you're just
> measuring contention between threads for the CPU since they're all
> working on different arenas.
> 
> Maybe one way to guarantee such contention is a test with one thread
> that allocates on an arena and another thread that frees from the same
> arena.  I don't think the current benchmark does that.

I would really like to see more attention to this usage case (allocate
in one thread, free in another). It's an idiomatic msg/data-passing
strategy and probably the least complex in most cases, and it's a
shame if people are avoiding it for performance reasons.

Rich