From 6b10c6c9fbfe754e8482efb8c8b84f8e40c0f2eb Mon Sep 17 00:00:00 2001 From: Fengguang Wu Date: Tue, 16 Oct 2007 01:24:34 -0700 Subject: [PATCH] readahead: basic support of interleaved reads This is a simplified version of the pagecache context based readahead. It handles the case of multiple threads reading on the same fd and invalidating each others' readahead state. It does the trick by scanning the pagecache and recovering the current read stream's readahead status. The algorithm works in a opportunistic way, in that it does not try to detect interleaved reads _actively_, which requires a probe into the page cache (which means a little more overhead for random reads). It only tries to handle a previously started sequential readahead whose state was overwritten by another concurrent stream, and it can do this job pretty well. Negative and positive examples(or what you can expect from it): 1) it cannot detect and serve perfect request-by-request interleaved reads right: time stream 1 stream 2 0 1 1 1001 2 2 3 1002 4 3 5 1003 6 4 7 1004 8 5 9 1005 Here no single readahead will be carried out. 2) However, if it's two concurrent reads by two threads, the chance of the initial sequential readahead be started is huge. Once the first sequential readahead is started for a stream, this patch will ensure that the readahead window continues to rampup and won't be disturbed by other streams. time stream 1 stream 2 0 1 1 2 2 1001 3 3 4 1002 5 1003 6 4 7 5 8 1004 9 6 10 1005 11 7 12 1006 13 1007 Here stream 1 will start a readahead at page 2, and stream 2 will start its first readahead at page 1003. From then on the two streams will be served right. Cc: Rusty Russell Signed-off-by: Fengguang Wu Cc: Rusty Russell Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/readahead.c | 33 +++++++++++++++++++++++---------- 1 file changed, 23 insertions(+), 10 deletions(-) diff --git a/mm/readahead.c b/mm/readahead.c index 4a58befbde4..fd588ffc508 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -379,6 +379,29 @@ ondemand_readahead(struct address_space *mapping, offset, req_size, 0); } + /* + * Hit a marked page without valid readahead state. + * E.g. interleaved reads. + * Query the pagecache for async_size, which normally equals to + * readahead size. Ramp it up and use it as the new readahead size. + */ + if (hit_readahead_marker) { + pgoff_t start; + + read_lock_irq(&mapping->tree_lock); + start = radix_tree_next_hole(&mapping->page_tree, offset, max+1); + read_unlock_irq(&mapping->tree_lock); + + if (!start || start - offset > max) + return 0; + + ra->start = start; + ra->size = start - offset; /* old async_size */ + ra->size = get_next_ra_size(ra, max); + ra->async_size = ra->size; + goto readit; + } + /* * It may be one of * - first read on start of file @@ -390,16 +413,6 @@ ondemand_readahead(struct address_space *mapping, ra->size = get_init_ra_size(req_size, max); ra->async_size = ra->size > req_size ? ra->size - req_size : ra->size; - /* - * Hit on a marked page without valid readahead state. - * E.g. interleaved reads. - * Not knowing its readahead pos/size, bet on the minimal possible one. - */ - if (hit_readahead_marker) { - ra->start++; - ra->size = get_next_ra_size(ra, max); - } - readit: return ra_submit(ra, mapping, filp); } -- 2.20.1