Re: [PATCH RFC] introduce dl_iterate_phdr_parallel

[Torvald Riegel <triegel at redhat dot com>]

On Mon, 2016-08-01 at 23:07 +0300, Gleb Natapov wrote:
> On Mon, Aug 01, 2016 at 09:46:35PM +0200, Torvald Riegel wrote:
> > If we built something custom for this and are willing to make the
> > wrlock / exclusive-access case much more costly, we can decrease this
> > overhead.  This could be roughly similar to one lock per thread or a set
> > of rwlocks as you mentioned, but with less space overhead.
> > 
> IMO space overhead is negligible. More efficient rwlock is, of course,
> better and can be useful in many more places. If you have something to
> test I am willing to do so, but if custom rwlock will take time to
> materialize we may start with lock array and change it later. The lock
> array is not part of the interface, but implementation detail. What I
> would like to avoid is stalling the afford while waiting for something
> better. Exception scalability is very pressing issue for us.

I haven't looked at the workload in detail, and so can't say off-hand
what would be best and how long it would take.  Is there a summary
describing the synchronization that is necessary?  Once I know that, I
can give a closer estimate.  It could be simple to do, so it might be
worth having a look before trying the (rw?)lock-array approach.

The POSIX rwlock's complexity is partly due to having to support the
different modes (eg, reader preference), so a custom rwlock for a
specific use case could be much simpler.  I would guess that it looks
more like reference counting, or if the snapshots of the readers are
suitable, it could also be seqlock based (and thus require no memory
modification by readers at all).