Re: [boost] [fiber] new version in vault

Subject: Re: [boost] [fiber] new version in vault
From: Anthony Williams (anthony.ajw_at_[hidden])
Date: 2009-12-01 06:38:22

Helge Bahmann <hcb_at_[hidden]> writes:

> On Mon, 30 Nov 2009, Phil Endecott wrote:
>
>> My work on this was backed up with extensive benchmarking,
>> disassembly of the generated code, and other evaluation.    You can
>> find some of the results in the list archive from about two years
>> ago.  There are many different types of system with different
>> characteristics (uniprocessor vs multiprocessor, two threads vs
>> 10000 threads, etc etc).  Two particular cases that I'll mention
>> are:
>
> I guess this is the code you used for testing?
>
>   https://svn.chezphil.org/mutex_perf/trunk
>
> I would say that your conclusions are valid for ARM only (I don't know
> the architecture or libc peculiarities), for x86 there are some
> subtleties which IMHO invalidate the comparison.
>
> Your spinlock implementation defers to __sync_lock_test_and_set, which
> in turn generates an "xchgl" instruction, and NOT an "lock xchgl"
> instruction (yes, these gcc primitives are tricky which is why I avoid
> them). 

On x86 these are equivalent --- the LOCK prefix is automatically
asserted for XCHG. See the XCHG instruction docs in the Intel manual
volumne 2B.

Anthony
-- 
Author of C++ Concurrency in Action     http://www.stdthread.co.uk/book/
just::thread C++0x thread library             http://www.stdthread.co.uk
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