Re-" Often, RW locks aren't a win because maintaining just the read
locks (without any writers) introduces contention at the hardware level, and
parallelism does not increase all that much as a result. " - Yes, RWlock has a
small price compared to no lock (it still stays in user space and does not
require any syscall if no writer is in), but is a significant improvement
compared to straight forward mutex, which even in case of all readers will put
a bunch of contexts to sleep on the futex_wait().
Igor.
-----Original Message-----
From: Salz, Rich
Sent: Wednesday, June 11, 2014 8:26 AM
To: openssl-dev@openssl.org
Cc: Levin, Igor
Subject: RE: Locking inefficiency
> What kinds of operations are protected by read locks?
Looking at almost any of the global data structures, such as error tables, OID
tables, and so on.
>Often, RW locks aren't a win because maintaining just the read locks (without
>any writers) introduces contention at the hardware level, and parallelism does
>not increase all that much as a result. Paul McKenney's dissertation on RCU
>has some examples.
We've monitored one of our applications, an SSL-terminating HTTP proxy server,
under load. Of all the mutexes (futex, actually) in the system, the "error"
lock is the most highly contended one. I'll see about posting some statistics.
/r$
--
Principal Security Engineer
Akamai Technologies, Cambridge, MA
IM: rs...@jabber.me<mailto:rs...@jabber.me>; Twitter: RichSalz
