Re: [PATCH RFC] introduce dl_iterate_phdr_parallel

[Florian Weimer <fweimer at redhat dot com>]

On 08/01/2016 09:46 PM, Torvald Riegel wrote:
> The new rwlock is built so that it supports process-shared usage, which
> means that we have to put everything into struct pthread_rwlock_t.  This
> will lead to contention if you rdlock it frequently from many threads.
> There is potential for tuning there because we haven't looked closely at
> adding back-off in the CAS loop (and if you tested on an arch without
> direct HW support for fetch-add, the CAS loop used instead of that might
> also be suboptimal).

The rwlock doesn't eliminate the contention at the hardware level.

If that causes a performance issue, we could reuse Ingo Molnar's brlock 
approach: per-thread, readers acquire their own lock, writers acquire 
the locks of all threads.  This is fairly efficient in the read case 
(and I suspect you can't get much better than that in a non-managed run 
tine), but the write case is obviously extremely costly.  This could be 
the right trade-off here, though.

Florian