[PATCH] readahead: multiple performance fixes

From: Ram Pai <linuxram@us.ibm.com>

I have enclosed a patch that fixes a bunch of performance bugs in the
readahead code.

Below is a brief summary of the problems noticed and the proposed fixes
with some results:
   
 Problem 1:  Readahead code closes the readahead window and goes into
 slowread path, if a file is accessed the first time at an offset
 notequal to zero.  In the case of databases(especially in db2), a file
 may not be accessed at offset 0 the first time though the i/o's are
 sequential.

 Fix to Problem 1:
          min = get_min_readahead(ra);
         orig_next_size = ra-next_size;

 -       if (ra-next_size == 0 && offset == 0) {
 +       if (ra-next_size == 0) {

------------------------------------------------------------------------

 ------------------------------------------------------------------------
 Problem 2: After fixing Problem, the readahead window still does not
 open up the first time, if all the pages requested are already in the
 page cache. This time the window closes because of pagecache hits
 instead of misses. To fix this we put in these changes.

 -               check_ra_success(ra, ra-size, actual, orig_next_size);
 +               if(!first_access) {
 +                       check_ra_success(ra, ra-size, actual, orig_next_size);
 +               }

 ------------------------------------------------------------------------

 ------------------------------------------------------------------------
 Problem 3: In the case of large random reads, the readahead window is
 read in, the moment there is a hit in the active window. And it turns
 out that in most of the cases the readahead window gets scrapped,
 because the next large random read does not even touch any of the pages
 in that readahead window. We fixed this by introducing lazy readahead.
 Basically we wait till the last page in the active window gets a hit.
 And once the last page is hit, the readahead window is then read in.
 This fix gave a tremendous boost in the performance.
 To fix this the changes we put in were:

                 /*
                  * This read request is within the current window.  It is time
                  * to submit I/O for the ahead window while the application is
                  * crunching through the current window.
                  */
 -               if (ra-ahead_start == 0) {
 +               if (ra-ahead_start == 0 && offset == (ra-start + ra-size -1)) {

 ------------------------------------------------------------------------

 ------------------------------------------------------------------------
 Problem 4:
       If the request page does not fall in the active window and is not
the  first page of the read ahead window, we scrap both the active
window and the readahaed window and read in the active window. But it
turns out that we read in a lot of pages in the active window based on
the size of the 'projected readahead window size' (the next_size
variable). And we end up using part of the active window and waste the
remaining. We put  in a fix where we read in just as many pages in the
active window based on the number of pages used in the recent past.
 Again this gave us another big boost in performance and ended up
beating the performance of aio patch on a DSS workload.

 The fix to this is:
                  * ahead window and get some I/O underway for the new
                  * current window.
                  */
 +               if (!first_access && preoffset = ra-start &&
 +                               preoffset < (ra-start + ra-size)) {
 +                       ra-size = preoffset - ra-start + 2;
 +               } else {
 +                       ra-size = ra-next_size;

 ------------------------------------------------------------------------

 ------------------------------------------------------------------------
 Problem 5:
 With all the above fixes there is very low chance that the readahead
 window shall close.  But however if it does, we found that the slow
 read  path is really slow.  Any loss of sequentiality in the slow read
 path is penalized heavily by closing the window back to zero. So we
 fixed this  by decreasing the window size by one anytime we loose
 sequentiality and  increasing in by 1 if we didn't.

                 if (offset != ra-prev_page + 1) {
 -                       ra-size = 0;            /* Not sequential */
 +                       ra-size = ra-size?ra-size-1:0; /*Notsequential */

 ------------------------------------------------------------------------

With the above set of fixes we got about 28% improvement in DSS workload
which is about 5% more than what we got with the suparna's aio patch.

This patch compared equivalent to suparna's aio patch with aio-stress
run.

It fared better than aio patch for large random io.

We are yet to run a bunch of other benchmarks to evaluate this patch.
We would like to get your inputs on this patch and any suggestions you
may have to improve it. I have enclosed a patch with all these changes
along with some changes to the comments that reflect the new behaviour.
NOTE: the above patch reverts suparna's aio patch.

mm/filemap.c

@@ -587,13 +587,22 @@ void do_generic_mapping_read(struct address_space *mapping,
 			     read_actor_t actor)
 {
 	struct inode *inode = mapping->host;
-	unsigned long index, offset;
+	unsigned long index, offset, last;
 	struct page *cached_page;
 	int error;
 
 	cached_page = NULL;
 	index = *ppos >> PAGE_CACHE_SHIFT;
 	offset = *ppos & ~PAGE_CACHE_MASK;
+	last = (*ppos + desc->count) >> PAGE_CACHE_SHIFT;
+
+	/*
+	 * Let the readahead logic know upfront about all
+	 * the pages we'll need to satisfy this request
+	 */
+	for (; index < last; index++)
+		page_cache_readahead(mapping, ra, filp, index);
+	index = *ppos >> PAGE_CACHE_SHIFT;
 
 	for (;;) {
 		struct page *page;
@@ -612,7 +621,6 @@ void do_generic_mapping_read(struct address_space *mapping,
 		}
 
 		cond_resched();
-		page_cache_readahead(mapping, ra, filp, index);
 
 		nr = nr - offset;
 find_page:

mm/readahead.c

@@ -347,6 +347,8 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
 	unsigned min;
 	unsigned orig_next_size;
 	unsigned actual;
+	int first_access=0;
+	unsigned long preoffset=0;
 
 	/*
 	 * Here we detect the case where the application is performing
@@ -370,16 +372,18 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
 	min = get_min_readahead(ra);
 	orig_next_size = ra->next_size;
 
-	if (ra->next_size == 0 && offset == 0) {
+	if (ra->next_size == 0) {
 		/*
-		 * Special case - first read from first page.
+		 * Special case - first read.
 		 * We'll assume it's a whole-file read, and
 		 * grow the window fast.
 		 */
+		first_access=1;
 		ra->next_size = max / 2;
 		goto do_io;
 	}
 
+	preoffset = ra->prev_page;
 	ra->prev_page = offset;
 
 	if (offset >= ra->start && offset <= (ra->start + ra->size)) {
@@ -439,20 +443,44 @@ page_cache_readahead(struct address_space *mapping, struct file_ra_state *ra,
 		 * ahead window and get some I/O underway for the new
 		 * current window.
 		 */
+		if (!first_access && preoffset >= ra->start &&
+				preoffset < (ra->start + ra->size)) {
+			 /* Heuristic:  If 'n' pages were
+			  * accessed in the current window, there
+			  * is a high probability that around 'n' pages
+			  * shall be used in the next current window.
+			  *
+			  * To minimize lazy-readahead triggered
+			  * in the next current window, read in
+			  * an extra page.
+			  */
+			ra->next_size = preoffset - ra->start + 2;
+		}
 		ra->start = offset;
 		ra->size = ra->next_size;
 		ra->ahead_start = 0;		/* Invalidate these */
 		ra->ahead_size = 0;
 		actual = do_page_cache_readahead(mapping, filp, offset,
 						 ra->size);
+		if(!first_access) {
+			/*
+			 * do not adjust the readahead window size the first
+			 * time, the ahead window might get closed if all
+			 * the pages are already in the cache.
+			 */
 			check_ra_success(ra, ra->size, actual, orig_next_size);
+		}
 	} else {
 		/*
 		 * This read request is within the current window.  It is time
 		 * to submit I/O for the ahead window while the application is
-		 * crunching through the current window.
+		 * about to step into the ahead window.
+		 * Heuristic: Defer reading the ahead window till we hit
+		 * the last page in the current window. (lazy readahead)
+		 * If we read in earlier we run the risk of wasting
+		 * the ahead window.
 		 */
-		if (ra->ahead_start == 0) {
+		if (ra->ahead_start == 0 && offset == (ra->start + ra->size -1)) {
 			ra->ahead_start = ra->start + ra->size;
 			ra->ahead_size = ra->next_size;
 			actual = do_page_cache_readahead(mapping, filp,
@@ -488,7 +516,7 @@ void handle_ra_miss(struct address_space *mapping,
 		const unsigned long max = get_max_readahead(ra);
 
 		if (offset != ra->prev_page + 1) {
-			ra->size = 0;			/* Not sequential */
+			ra->size = ra->size?ra->size-1:0; /* Not sequential */
 		} else {
 			ra->size++;			/* A sequential read */
 			if (ra->size >= max) {		/* Resume readahead */