On Wed, 2007-01-24 at 22:22 +0800, Aubrey Li wrote: > On 1/24/07, Peter Zijlstra <[EMAIL PROTECTED]> wrote: > > On Tue, 2007-01-23 at 16:49 -0800, Christoph Lameter wrote: > > > This is a patch using some of Aubrey's work plugging it in what is IMHO > > > the right way. Feel free to improve on it. I have gotten repeatedly > > > requests to be able to limit the pagecache. With the revised VM statistics > > > this is now actually possile. I'd like to know more about possible uses of > > > such a feature. > > > > > > > > > > > > > > > It may be useful to limit the size of the page cache for various reasons > > > such as > > > > > > 1. Insure that anonymous pages that may contain performance > > > critical data is never subject to swap. > > > > This is what we have mlock for, no? > > > > > 2. Insure rapid turnaround of pages in the cache. > > > > This sounds like we either need more fadvise hints and/or understand why > > the VM doesn't behave properly. > > > > > 3. Reserve memory for other uses? (Aubrey?) > > > > He wants to make a nommu system act like a mmu system; this will just > > never ever work. > > Nope. Actually my nommu system works great with some of patches made by us. > What let you think this will never work?
Because there are perfectly valid things user-space can do to mess you up. I forgot the test-case but it had something to do with opening a million files, this will scatter slab pages all over the place. Also, if you cycle your large user-space allocations a bit unluckily you'll also fragment it into oblivion. So you can not guarantee it will not fragment into smithereens stopping your user-space from using large than page size allocations. If your user-space consists of several applications that do dynamic memory allocation of various sizes its a matter of (run-) time before things will start failing. If you prealloc a large area at boot time (like we now do for hugepages) and use that for user-space, you might 'reset' the status quo by cycling the whole of userspace. > > Memory fragmentation is a real issue not some gimmick > > thought up by the hardware folks to sell these mmu chips. > > > I totally disagree. Memory fragmentations is the issue not only on > nommu, it's also on mmu chips. That's not the reason mmu chips can be > sold. For MMU enabled chips these fragmentation issues (at the page allocation level) will never reach (regular - !hugepages) user-space. Exactly because of the MMU, it will make things virtually contiguous. Yes, there are problem in kernel space, esp. when we want to use huge pages. - 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/
