On Tue, 10 Jul 2007 17:03:26 -0400 Chris Mason <chris.mason@xxxxxxxxxx> wrote: > This patch aims to demonstrate one way to replace buffer heads with a > few extent trees. Buffer heads provide a few different features: > > 1) Mapping of logical file offset to blocks on disk > 2) Recording state (dirty, locked etc) > 3) Providing a mechanism to access sub-page sized blocks. > > This patch covers #1 and #2, I'll start on #3 a little later next > week. > Well, almost. I decided to try out an rbtree instead of the radix, which turned out to be much faster. Even though individual operations are slower, the rbtree was able to do many fewer ops to accomplish the same thing, especially for merging extents together. It also uses much less ram. This code still has lots of room for optimization, but it comes in at around 2-5% more cpu time for ext2 streaming reads and writes. I haven't done readpages or writepages yet, so this is more or less a worst case setup. I'm comparing against ext2 with readpages and writepages disabled. The new code has the added benefit of passing fsx-linux, and not triggering MCE's on my poor little test box. The basic idea is to store state in byte ranges in an rbtree, and to mirror that state down into individual pages. This allows us to store arbitrary state outside of the page struct, so we could include the pid of the process that dirtied a page range for cfq purposes. The example readpage and writepage code is probably the easiest way to understand the basic API. A separate rbtree stores a mapping of byte offset in the file to byte offset on disk. This allows the filesystem to fill in mapping information in bulk, and reduces the number of metadata lookups required to do common operations. Because the state and mapping information are separate from the page, pages can come and go and their corresponding metadata can still be cached (the current code drops mappings as the last page corresponding to that mapping disappears). Two patches follow, the core extent_map implementation and a sample user (ext2). This is pretty basic, implementing prepare/commit_write, read/writepage and a few other funcs to exercise the new code. Longer term, it should fit in with Nick's other extent work instead of prepare/commit_write. My patch sets page->private to 1, really for no good reason. It is just a debugging aid I was using to make sure the page took the right path down the line. If this catches on, we might set it to a magic value so you can if (ExtentPage(page)) or just leave it as null. -chris - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
