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Date: Fri, 29 Sep 2006 11:27:25 -0500 From: "Ram Gupta" <ram.gupta5@...il.com> To: "Andrew Morton" <akpm@...l.org> Cc: "linux mailing-list" <linux-kernel@...r.kernel.org> Subject: [PATCH]doc: Added explaination for splice system call This patch adds documentation for splice system call. Please apply Signed-off-by: Ram Gupta<r.gupta@...ronautics.com> ------- diff -uprN -X linux-2.6.18-doc/Documentation/dontdiff linux-2.6.18/Documentation/splice.txt linux-2.6.18-doc/Documentation/splice.txt --- linux-2.6.18/Documentation/splice.txt 1969-12-31 19:00:00.000000000 -0500 +++ linux-2.6.18-doc/Documentation/splice.txt 2006-09-29 10:54:11.000000000 -0500 @@ -0,0 +1,57 @@ + Splice system call + ---------------------------- +A splice() is a system call mechanism to do i/o from one file to another file +in kernel space without doing copying from/to user space.It is a way of +improving I/O performance. The splice system call avoids all data copy from +user space to kernel space & vice versa. It reads from the specified offset +from the input file & writes to a pipe in the kernel space. There is no copying +of data to user space.Then it can be called to write the data from the pipe +to the output file at the specified/current offset. + +splice() works by using the pipe buffer mechanism to open a file descriptor +for a data source and another for a data sink then by using splice() it can +join the two together. In other words, splice() work on a kernel buffer that +the user has control over and moves data to/from the buffer from/to an +arbitrary file descriptor. Specifying offset with pipe is an error as usual. +If no offset is specified with an input/output file descriptor then the +current offset will be assumed to be the offset specified. Currently one of +the file descriptor must be pipe otherwise it is an error. +This system call was added in 2.6.17 kernel + +######################################## +Overview of reading from a file to pipe +######################################## +* It does the read ahead if there is more than one page + +* It tries to find the nr_pages from the input file mapping (contiguous + if possible) + +* If we got less number of contiguous pages than needed then find the other + pages or allocate new page. + +* If the page is not uptodate then we may need to start io on that page. But + the page may be under i/o already. + +* If needed start io if we are not in non-block mode. + +* Set up the pages in the buffers of the pipe. + +################################################ +Overview of writing from pipe to the output file +################################################# + +* Make sure the page is uptodate + +* Steal the page from the page cache if the flag SPLICE_F_MOVE is set & add the + page to the page cache of the file mapping. + +* Otherwise find the page of the mapping for the offset & make sure that page + is uptodate. + +* Prepare the page to be written in the file + +* balance dirty memory state + +* If file or inode is SYNC and we actually wrote some data, sync it. + +Ram Gupta<r.gupta@...ronautics.com> Thanks Ram Gupta - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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