From mboxrd@z Thu Jan 1 00:00:00 1970 From: Matthew Wilcox Date: Fri, 08 Nov 2002 19:19:07 +0000 Subject: Re: [Linux-ia64] reader-writer livelock problem Message-Id: List-Id: References: In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org On Fri, Nov 08, 2002 at 12:05:30PM -0600, Van Maren, Kevin wrote: > Absolutely you should minimize the locking contention. > However, that isn't always possible, such as when you > have 64 processors contending on the same resource. if you've got 64 processors contending on the same resource, maybe you need to split that resource up so they can have a copy each. all that cacheline bouncing can't do your numa boxes any good. > With the current kernel, the trivial example with reader/ > writer locks was having them all call gettimeofday(). i hear x86-64 has a lockless gettimeofday. maybe that's the solution. > But try having 64 processors fstat() the same file, > which I have also seen happen (application looping, > waiting for another process to finish setting up the > file so they can all mmap it). umm.. the call trace: sys_fstat |-> vfs_fstat | |-> fget | |-> read_lock(&files->file_lock) | |-> vfs_getattr | |-> inode->i_op->getattr | |-> generic_fillattr |-> cp_new_stat64 |-> memset |-> copy_to_user so you're talking about contention on files->file_lock, right? it's really not the kernel's fault that your app is badly written. that lock's private to process & children, so it's not like another application can hurt you. > What MCS locks do is they reduce the number of times > the cacheline has to be flung around the system in > order to get work done: they "scale" much better with > the number of processors: O(N) instead of O(N^2). yes, but how slow are they in the uncontended case? -- Revolutions do not require corporate support.