lkml.org 
[lkml]   [2006]   [Aug]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: Why Semaphore Hardware-Dependent?
Date
Andi Kleen <ak@suse.de> wrote:

> > > BTW maybe it would be a good idea to switch the wait list to a hlist,
> > > then the last user in the queue wouldn't need to
> > > touch the cache line of the head. Or maybe even a single linked
> > > list then some more cache bounces might be avoidable.
> >
> > You need a list_head to get O(1) push at one end and O(1) pop at the other.
>
> The poper should know its node address already because it's on its own stack.

No. The popper (__rwsem_do_wake) runs in the context of up_xxxx(), not
down_xxxx(). Remember: up() may need to wake up several processes if there's
a batch of readers at the front of the queue.

Remember also: rwsems, unlike mutexes, are completely fair.

> > In addition a singly-linked list makes interruptible ops non-O(1) also.
>
> When they are interrupted I guess? Hardly a problem to make that slower.

Currently interruptible rwsems are not available, but that may change, and
whilst I agree making it slower probably isn't a problem, it's still a point
that has to be considered.

David
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-08-29 20:59    [W:0.074 / U:0.656 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site