[PATCH 3/3] readahead: scale max readahead size depending on memory size

[Peter Zijlstra]

Scale the default max readahead size with the system memory size.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 block/ll_rw_blk.c  |    2 +-
 include/linux/fs.h |    1 +
 mm/readahead.c     |   32 ++++++++++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 1 deletion(-)

Index: linux-2.6/block/ll_rw_blk.c
===================================================================
--- linux-2.6.orig/block/ll_rw_blk.c
+++ linux-2.6/block/ll_rw_blk.c
@@ -208,7 +208,7 @@ void blk_queue_make_request(request_queu
 	blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
 	blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
 	q->make_request_fn = mfn;
-	q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
+	bdi_ra_init(&q->backing_dev_info);
 	q->backing_dev_info.state = 0;
 	q->backing_dev_info.capabilities = BDI_CAP_MAP_COPY;
 	blk_queue_max_sectors(q, SAFE_MAX_SECTORS);
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h
+++ linux-2.6/include/linux/fs.h
@@ -1696,6 +1696,7 @@ extern long do_splice_direct(struct file
 
 extern void
 file_ra_state_init(struct file_ra_state *ra, struct address_space *mapping);
+extern void bdi_ra_init(struct backing_dev_info *bdi);
 extern loff_t no_llseek(struct file *file, loff_t offset, int origin);
 extern loff_t generic_file_llseek(struct file *file, loff_t offset, int origin);
 extern loff_t remote_llseek(struct file *file, loff_t offset, int origin);
Index: linux-2.6/mm/readahead.c
===================================================================
--- linux-2.6.orig/mm/readahead.c
+++ linux-2.6/mm/readahead.c
@@ -42,6 +42,38 @@ file_ra_state_init(struct file_ra_state 
 }
 EXPORT_SYMBOL_GPL(file_ra_state_init);
 
+static unsigned long ra_pages;
+
+static __init int readahead_init(void)
+{
+	/*
+	 * Scale the max readahead window with system memory
+	 *
+	 *   64M:   128K
+	 *  128M:   180K
+	 *  256M:   256K
+	 *  512M:   360K
+	 *    1G:   512K
+	 *    2G:   724K
+	 *    4G:  1024K
+	 *    8G:  1448K
+	 *   16G:  2048K
+	 */
+	ra_pages = int_sqrt(totalram_pages/16);
+	if (ra_pages > (2 << (20 - PAGE_SHIFT)))
+		ra_pages = 2 << (20 - PAGE_SHIFT);
+
+	return 0;
+}
+
+subsys_initcall(readahead_init);
+
+void bdi_ra_init(struct backing_dev_info *bdi)
+{
+	bdi->ra_pages = ra_pages;
+}
+EXPORT_SYMBOL(bdi_ra_init);
+
 #define list_to_page(head) (list_entry((head)->prev, struct page, lru))
 
 /**

--

-
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