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Re: why does rwlock prefer readers by default?
- From: Torvald Riegel <triegel at redhat dot com>
- To: Rich Felker <dalias at libc dot org>
- Cc: GLIBC Devel <libc-alpha at sourceware dot org>
- Date: Wed, 07 May 2014 22:09:05 +0200
- Subject: Re: why does rwlock prefer readers by default?
- Authentication-results: sourceware.org; auth=none
- References: <1399458831 dot 32485 dot 12625 dot camel at triegel dot csb> <20140507185121 dot GZ26358 at brightrain dot aerifal dot cx>
On Wed, 2014-05-07 at 14:51 -0400, Rich Felker wrote:
> On Wed, May 07, 2014 at 12:33:51PM +0200, Torvald Riegel wrote:
> > POSIX makes it an implementation-defined choice whether readers or
> > writers are preferred. Our current implementation's default is that
> > readers are to be preferred. I couldn't find the rationale for this;
> > does anybody know what it was?
> > Otherwise, if this was an arbitrary choice, what do you all think the
> > default should be? Can we change it? Should we change it to preferring
> > writers?
> As far as I know, there is no way to prefer writers but allow
> recursive locking by readers (which the standard requires be allowed)
> without unbounded memory usage (to track which threads already own a
> read lock). The problem is that you can't distinguish a new reader
> from an existing reader performing a recurive lock and thus you have
> to allow both, even if a writer is waiting. Please correct me if I'm
> wrong on this.
Yes you need space to track which readers have acquired which rwlock.
At some number of recursive/rwlocks, there will be a runtime overhead.
Irrespective of that, there's another case where we can prefer readers
or writers, as I've described elsewhere in the thread.