Re: rwlock changes

[Christoph Egger <Christoph_Egger%gmx.de@localhost>]

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Andrew Doran wrote:
| Hi,
|
| http://www.netbsd.org/~ad/rwlock.diff
|
| This diff does the following:
|
| - Use atomic ops directly, since rwlocks work the same way on all
platforms.
| - Try to make it a bit more cache efficient, and use branch hints.
| - Fix a bug in rw_downgrade() where the turnstile lock was not released.
| - Remove a couple of redundant assertions.
| - Use atomic_swap instead of atomic_cas in one place where it's safe.
| - After acquiring turnstile lock in rw_vector_enter, check if owner is
|   running again and spin if so.
|
| It also changes the handoff/release algorithm to work like this:
|
|          * If we are releasing a write lock, then prefer to wake all
|          * outstanding readers.  Otherwise, wake one writer if there
|          * are outstanding readers, or all writers if there are no
|          * pending readers.  If waking one specific writer, the writer
|          * is handed the lock here.  If waking multiple writers, we
|          * set WRITE_WANTED to block out new readers, and let them
|          * do the work of acquring the lock in rw_vector_enter().
|
| So, direct handoff is only done when there are readers involved (wakeing
| readers, or readers pending on the lock). Otherwise threads being awoken
| must acquire the lock themselves, making it work like an adaptive mutex in
| that case. It weakens the defence against starvation slightly but I'm
| fairly confident that it shouldn't be a problem.
|
| The sum of these changes is a reduction of 4% in build time on an 8-core
| machine. There is less idle time during the build because the windows
where
| a rwlock can be held but the owner is not running is reduced.

What do you think about "Sequential locks" ?

http://en.wikipedia.org/wiki/Sequential_Lock


Christoph

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