On Wed 07-08-13 11:00:52, Andy Lutomirski wrote: > On Wed, Aug 7, 2013 at 10:40 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote: > > On 08/07/2013 06:40 AM, Jan Kara wrote: > >> One question before I look at the patches: Why don't you use fallocate() > >> in your application? The functionality you require seems to be pretty > >> similar to it - writing to an already allocated block is usually quick. > > > > One problem I've seen is that it still costs you a fault per-page to get > > the PTEs in to a state where you can write to the memory. MADV_WILLNEED > > will do readahead to get the page cache filled, but it still leaves the > > pages unmapped. Those faults get expensive when you're trying to do a > > couple hundred million of them all at once. > > I have grand plans to teach the kernel to use hardware dirty tracking > so that (some?) pages can be left clean and writable for long periods > of time. This will be hard. Right that will be tough... Although with your application you could require such pages to be mlocked and then I could imagine we would get away at least from problems with dirty page accounting. > Even so, the second write fault to a page tends to take only a few > microseconds, while the first one often blocks in fs code. So you wrote blocks are already preallocated with fallocate(). If you also preload pages in memory with MADV_WILLNEED is there still big difference between the first and subsequent write fault? > (mmap_sem is a different story, but I see it as a separate issue.) Yeah, agreed. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
