-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 1/31/2014 8:53 AM, Theodore Ts'o wrote: > Something that we might need to go to in the future is instead of > using mmap(), to maintain our own explicit buffer cache inside > unix_io, and use direct I/O to avoid caching the disk blocks > twice. Then when we use a single-threaded disk prefetcher, managed > by the unix_io, it will know when a particular I/O request has > completed, and more importantly, if there is a synchronous read > request coming in from main body of the program, it can stop > prefetching and allow the higher priority read to complete. We can > also experiment with how many threads might make sense --- even > with an HDD, using multiple threads so that we can take advantage > of NCQ might still be a win. Why build your own cache instead of letting the kernel take care of it? I believe the IO elevator already gives preferential treatment to blocking reads so just using readahead() to prefetch and sticking with plain old read() should work nicely. > Finally, if we are managing our own buffer cache, we should > consider adding a bforget method to the I/O manager. That way > e2fsck can give hints to the caching layer that a block isn't > needed any more. If it is in the cache, it can be dropped, to free > memory, and if it is still on the to-be-prefetched list it should > also be dropped. (Of course, if a block is on the to-be-prefetched > list, and a synchronous read request comes in for that block, we > should have dropped it from the to-be-prefetched list at that > point.) The main use for having a bforget method is for the most > part, once we are done scanning a non-directory extent tree block, > we won't be needing it again. Good idea, but this also could just be translated to posix_fadvise to have the kernel drop the pages from the cache. -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.17 (MingW32) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iQEcBAEBAgAGBQJTD2/8AAoJEI5FoCIzSKrwN9QIAKKyOssDAoy6Zab7w+j6aXdH f8Lzw/+WWevPpvX1o9SakQXVYejkiZBcEhJ/NrtQoDzQjesrERpKIj7m+Yb2blaQ VYu9hj21ReVYngmmOlUHL9LJJtfwGptyO3SGvuSZv3NooMjEu51tCXxHLxONaYAb SqvRBVH57E7hY1HckKTbD14YleKn/SD7peNIp1KP39zQw1soR83+oHh2qZ3Zaq7E fa/UG7tnIMcOjHghacr83LA7XW+sQYn8pDeAmw38SKe+uiHvcqjzych6lQlKLw19 pKdzERd8zaPsiS2y4og/RzWZHolohiDp67cWRncYKm3ai2w4KbU9KXlQHa/6Fy0= =orjY -----END PGP SIGNATURE----- -- 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
