Re: [RFC] Lock free fd lookup

[Chris Wright <chrisw@osdl.org>]

* Dipankar Sarma (dipankar@in.ibm.com) wrote:
> On Thu, Jul 15, 2004 at 10:22:53AM -0400, Jesse Barnes wrote:
> > On Wednesday, July 14, 2004 11:17 am, Chris Wright wrote:
> > > I'm curious, how much of the performance improvement is from RCU usage
> > > vs. making the basic syncronization primitive aware of a reader and
> > > writer distinction?  Do you have benchmark for simply moving to rwlock_t?
> > 
> > That's a good point.  Also, even though the implementation may be 'lockless', 
> > there are still a lot of cachelines bouncing around, whether due to atomic 
> > counters or cmpxchg (in fact the latter will be worse than simple atomics).
> 
> Chris raises an interesting issue. There are two ways we can benefit from
> lock-free lookup - avoidance of atomic ops in lock acquisition/release
> and avoidance of contention. The latter can also be provided by
> rwlocks in read-mostly situations like this, but rwlock still has
> two atomic ops for acquisition/release. So, in another
> thread, I have suggested looking into the contention angle. IIUC,
> tiobench is threaded and shares fd table. 

Given the read heavy assumption that RCU makes (supported by your
benchmarks), I believe that the comparison with RCU vs. current scheme
is unfair.  Better comparison is against rwlock_t, which may give a
similar improvement w/out the added complexity.  But, I haven't a patch
nor a benchmark, so it's all handwavy at this point.

thanks,
-chris
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