On Thu, Aug 8, 2013 at 3:18 AM, Jan Kara <jack@xxxxxxx> wrote: > 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. True. The nasty part will be all the code that assumes that the acts of un-write-protecting and dirtying are the same thing, for example __block_write_begin, which is why I don't really believe in my willwrite patches... > >> 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? I haven't measured it yet, because I suspect that my patches are rather buggy in their current form. But the idea is that fallocate will do the heavy lifting and give me a nice contiguous allocation, and the MADV_WILLNEED call will take about as long as the first write fault would have taken. Then the first write fault after MADV_WILLNEED will take about as long as the second write fault would have taken without it. --Andy -- 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
