On Wed, Oct 31, 2012 at 06:15:33PM -0700, Paul Turner wrote: > On Wed, Oct 31, 2012 at 3:56 PM, KOSAKI Motohiro > <kosaki.motohiro@xxxxxxxxx> wrote: > >>> > Allocator should call madvise(MADV_NOVOLATILE) before reusing for > >>> > allocating that area to user. Otherwise, accessing of volatile range > >>> > will meet SIGBUS error. > >>> > >>> Well, why? It would be easy enough for the fault handler to give > >>> userspace a new, zeroed page at that address. > >> > >> Note: MADV_DONTNEED already has this (nice) property. > > > > I don't think I strictly understand this patch. but maybe I can answer why > > userland and malloc folks don't like MADV_DONTNEED. > > > > glibc malloc discard freed memory by using MADV_DONTNEED > > as tcmalloc. and it is often a source of large performance decrease. > > because of MADV_DONTNEED discard memory immediately and > > right after malloc() call fall into page fault and pagesize memset() path. > > then, using DONTNEED increased zero fill and cache miss rate. > > > > At called free() time, malloc don't have a knowledge when next big malloc() > > is called. then, immediate discarding may or may not get good performance > > gain. (Ah, ok, the rate is not 5:5. then usually it is worth. but not everytime) > > > > Ah; In tcmalloc allocations (and their associated free-lists) are > binned into separate lists as a function of object-size which helps to > mitigate this. > > I'd make a separate more general argument here: > If I'm allocating a large (multi-kilobyte object) the cost of what I'm > about to do with that object is likely fairly large -- The fault/zero > cost a probably fairly small proportional cost, which limits the > optimization value. While I look at thread trial of Rik which is same goal while implementation is different, I found this number. https://lkml.org/lkml/2007/4/20/390 I believe optimiation is valuable. Of course, I need simillar testing for proving it. > > > > > In past, several developers tryied to avoid such situation, likes > > > > - making zero page daemon and avoid pagesize zero fill at page fault > > - making new vma or page flags and mark as discardable w/o swap and > > vmscan treat it. (like this and/or MADV_FREE) > > - making new process option and avoid page zero fill from page fault path. > > (yes, it is big incompatibility and insecure. but some embedded folks thought > > they are acceptable downside) > > - etc > > > > > > btw, I'm not sure this patch is better for malloc because current MADV_DONTNEED > > don't need mmap_sem and works very effectively when a lot of threads case. > > taking mmap_sem might bring worse performance than DONTNEED. dunno. > > MADV_VOLATILE also seems to end up looking quite similar to a > user-visible (range-based) cleancache. > > A second popular use-case for such semantics is the case of > discardable cache elements (e.g. web browser). I suspect we'd want to > at least mention these in the changelog. (Alternatively, what does a > cleancache-backed-fs exposing these semantics look like?) > It's a trial of John Stultz(http://lwn.net/Articles/518130/, there was another trial long time ago https://lkml.org/lkml/2005/11/1/384) and I want to expand the concept from file-backed page to anonymous page so this patch is a trial for anonymous page. So, usecase of my patch have focussed on malloc/free case. I hope both are able to be unified. > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>