On Wed, Apr 04, 2007 at 04:14:57PM +0200, Andrea Arcangeli wrote: > On Wed, Apr 04, 2007 at 04:55:32PM +0300, Dan Aloni wrote: > > How about applications that perform mmap() and R/W random-access on > > large *sparse* files? (e.g. a scientific app that uses a large sparse > > file as a big database look-up table). As I see it, these apps would > > need to keep track of what's sparse and what's not... > > That's not anonymous memory if those are read page faults on > _files_. I'm only talking about anonymous memory and > do_anonymous_page, i.e. no file data at all. In more clear words, the > only thing we're discussing here is char = malloc(1); *char. > > Your example _already_ allocates zeroed pagecache instead of the zero > page, so your example (random access over sparse files with mmap, be > it MAP_PRIVATE or MAP_SHARED no difference for reads) has never had > anything to do with the zero page. If something we could optimize your > example to _start_ using for the first time ever the ZERO_PAGE, it > would make more sense to use it to be mapped where the lowlevel fs > finds holes. ZERO_PAGE in do_anonymous_page instead doesn't make much > sense to me, but it has always been there as far as I can > remember. The thing is that it never hurted until the huge systems > with nightmare cacheline bouncing reported heavy stalls on some > testcase, which make it look like a DoS because of the ZERO_PAGE, > hence now that it hurts I guess it can go.
Oh, right. Thanks for clarifing. I should have figured it out before I sent that mail. To refine that example, you could replace the file with a large anonymous memory pool and a lot of swap space committed to it. In that case - with no ZERO_PAGE, would the kernel needlessly swap-out the zeroed pages? Perhaps it's an example too far-fetched to worth considering... -- Dan Aloni XIV LTD, http://www.xivstorage.com da-x (at) monatomic.org, dan (at) xiv.co.il - 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/
