On Wed, Oct 31, 2012 at 3:56 PM, KOSAKI Motohiro <kosaki.motoh...@gmail.com> 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. > > 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?) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
