From 46fc3e7b4e7233a0ac981ac9084b55217318d04d Mon Sep 17 00:00:00 2001 From: Fengguang Wu Date: Thu, 19 Jul 2007 01:47:57 -0700 Subject: [PATCH] readahead: add look-ahead support to __do_page_cache_readahead() Add look-ahead support to __do_page_cache_readahead(). It works by - mark the Nth backwards page with PG_readahead, (which instructs the page's first reader to invoke readahead) - and only do the marking for newly allocated pages. (to prevent blindly doing readahead on already cached pages) Look-ahead is a technique to achieve I/O pipelining: While the application is working through a chunk of cached pages, the kernel reads-ahead the next chunk of pages _before_ time of need. It effectively hides low level I/O latencies to high level applications. Signed-off-by: Fengguang Wu Cc: Steven Pratt Cc: Ram Pai Cc: Rusty Russell Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/readahead.c | 15 +++++++++------ 1 file changed, 9 insertions(+), 6 deletions(-) diff --git a/mm/readahead.c b/mm/readahead.c index 9861e883fe..88ea0f29aa 100644 --- a/mm/readahead.c +++ b/mm/readahead.c @@ -265,7 +265,8 @@ out: */ static int __do_page_cache_readahead(struct address_space *mapping, struct file *filp, - pgoff_t offset, unsigned long nr_to_read) + pgoff_t offset, unsigned long nr_to_read, + unsigned long lookahead_size) { struct inode *inode = mapping->host; struct page *page; @@ -278,7 +279,7 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp, if (isize == 0) goto out; - end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); + end_index = ((isize - 1) >> PAGE_CACHE_SHIFT); /* * Preallocate as many pages as we will need. @@ -301,6 +302,8 @@ __do_page_cache_readahead(struct address_space *mapping, struct file *filp, break; page->index = page_offset; list_add(&page->lru, &page_pool); + if (page_idx == nr_to_read - lookahead_size) + SetPageReadahead(page); ret++; } read_unlock_irq(&mapping->tree_lock); @@ -337,7 +340,7 @@ int force_page_cache_readahead(struct address_space *mapping, struct file *filp, if (this_chunk > nr_to_read) this_chunk = nr_to_read; err = __do_page_cache_readahead(mapping, filp, - offset, this_chunk); + offset, this_chunk, 0); if (err < 0) { ret = err; break; @@ -384,7 +387,7 @@ int do_page_cache_readahead(struct address_space *mapping, struct file *filp, if (bdi_read_congested(mapping->backing_dev_info)) return -1; - return __do_page_cache_readahead(mapping, filp, offset, nr_to_read); + return __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0); } /* @@ -404,7 +407,7 @@ blockable_page_cache_readahead(struct address_space *mapping, struct file *filp, if (!block && bdi_read_congested(mapping->backing_dev_info)) return 0; - actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read); + actual = __do_page_cache_readahead(mapping, filp, offset, nr_to_read, 0); return check_ra_success(ra, nr_to_read, actual); } @@ -449,7 +452,7 @@ static int make_ahead_window(struct address_space *mapping, struct file *filp, * @req_size: hint: total size of the read which the caller is performing in * PAGE_CACHE_SIZE units * - * page_cache_readahead() is the main function. If performs the adaptive + * page_cache_readahead() is the main function. It performs the adaptive * readahead window size management and submits the readahead I/O. * * Note that @filp is purely used for passing on to the ->readpage[s]() -- 2.39.5