On 06/10/2015 10:07 AM, Nils Goroll wrote:
On larger Linux machines, we have been running with spin locks replaced by
generic posix mutexes for years now. I personally haven't look at the code for
ages, but we maintain a patch which pretty much does the same thing still:
Ref: http://www.postgresql.org/message-id/4fede0bf.7080...@schokola.de
I understand that there are systems out there which have less efficient posix
mutex implementations than Linux (which uses futexes), but I think it would
still be worth considering to do away with the roll-your-own spinlocks on
systems whose posix mutexes are known to behave.
I have played with test code that isolates a stripped down version of
s_lock() and uses it with multiple threads. I then implemented multiple
different versions of that s_lock(). The results with 200 concurrent
threads are that using a __sync_val_compare_and_swap() to acquire the
lock and then falling back to a futex() is limited to about 500,000
locks/second. Spinning for 10 times and then doing a usleep(1000) (one
millisecond) gives me 25 million locks/second.
Note that the __sync_val_compare_and_swap() GCC built in seems identical
in performance with the assembler xchgb operation used by PostgreSQL
today on x84_64.
Regards, Jan
--
Jan Wieck
Senior Software Engineer
http://slony.info
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