On Mon, Apr 02, 2007 at 09:57:02PM -0700, Andrew Morton wrote: > On Tue, 03 Apr 2007 14:45:02 +1000 Rusty Russell <[EMAIL PROTECTED]> wrote: > > > On Mon, 2007-04-02 at 20:58 -0700, Andrew Morton wrote: > > > On Tue, 03 Apr 2007 13:44:45 +1000 Rusty Russell <[EMAIL PROTECTED]> > > > wrote: > > > > > > > > > > > I can never remember what the function to register to receive VM > > > > pressure > > > > is called. I have to trace down from __alloc_pages() to find it. > > > > > > > > It's called "set_shrinker()", and it needs Your Help. > > > > > > > > New version: > > > > 1) Don't hide struct shrinker. It contains no magic. > > > > 2) Don't allocate "struct shrinker". It's not helpful. > > > > 3) Call them "register_shrinker" and "unregister_shrinker". > > > > 4) Call the function "shrink" not "shrinker". > > > > 5) Rename "nr_to_scan" argument to "nr_to_free". > > > > > > No, it is actually the number to scan. This is >= the number of freed > > > objects. > > > > > > This is because, for better of for worse, the VM tries to balance the > > > scanning rate of the various caches, not the reclaiming rate. > > > > Err, ok, I completely missed that distinction. > > > > Does that mean the to function correctly every user needs some internal > > cursor so it doesn't end up scanning the first N entries over and over? > > > > If it wants to be well-behaved, and to behave as the VM expects, yes. > > There's an expectation that the callback will be performing some scan-based > aging operation and of course to do LRU (or whatever) aging, the callback > will need to remember where it was up to last time it was called. > > But it's just a guideline - callbacks could do something different but > in-the-spirit, I guess.
In XFS, one of the shrinkers cwthat gets registered calls causes all the xfsbufd's in the system to run and write back delayed write metadata - this can't be freed up until it is clean, and this is the only hook we have that can be used to trigger writeback on memory pressure. We need this because we can potentially have hundreds of megabytes of dirty metadata per XFS filesystem. IOW, the way the VM expects the shrinkers to work can be far, far away from what subsystems need the shrinker callbacks for.... Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
