Re: Random file I/O regressions in 2.6

[Andrew Morton <akpm@osdl.org>]

Ram Pai <linuxram@us.ibm.com> wrote:
>
> Sorry, If I am saying this again. I have checked the behaviour of the
>  readahead code using my user level simulator as well as running some
>  DSS benchmark and iozone benchmark. It generates a steady stream of
>  large i/o for large-random-reads and should not exhibit the bad behavior
>  that we are seeing.  I feel this bad behavior is because of interleaved
>  access by multiple thread. 

you're right - the benchmark has multiple threads issuing concurrent
pread()s against the same fd.  For some reason this mucks up the 2.6
readahead state more than 2.4's.

Putting a semaphore around do_generic_file_read() or maintaining the state
as below fixes it up.

I wonder if we should bother fixing this?  I guess as long as the app is
using pread() it is a legitimate thing to be doing, so I guess we should...



--- 25/mm/filemap.c~readahead-seralisation	2004-05-03 23:14:43.399947720 -0700
+++ 25-akpm/mm/filemap.c	2004-05-03 23:14:43.404946960 -0700
@@ -612,7 +612,7 @@ EXPORT_SYMBOL(grab_cache_page_nowait);
  * - note the struct file * is only passed for the use of readpage
  */
 void do_generic_mapping_read(struct address_space *mapping,
-			     struct file_ra_state *ra,
+			     struct file_ra_state *_ra,
 			     struct file * filp,
 			     loff_t *ppos,
 			     read_descriptor_t * desc,
@@ -622,6 +622,7 @@ void do_generic_mapping_read(struct addr
 	unsigned long index, offset;
 	struct page *cached_page;
 	int error;
+	struct file_ra_state ra = *_ra;
 
 	cached_page = NULL;
 	index = *ppos >> PAGE_CACHE_SHIFT;
@@ -644,13 +645,13 @@ void do_generic_mapping_read(struct addr
 		}
 
 		cond_resched();
-		page_cache_readahead(mapping, ra, filp, index);
+		page_cache_readahead(mapping, &ra, filp, index);
 
 		nr = nr - offset;
 find_page:
 		page = find_get_page(mapping, index);
 		if (unlikely(page == NULL)) {
-			handle_ra_miss(mapping, ra, index);
+			handle_ra_miss(mapping, &ra, index);
 			goto no_cached_page;
 		}
 		if (!PageUptodate(page))
@@ -752,6 +753,8 @@ no_cached_page:
 		goto readpage;
 	}
 
+	*_ra = ra;
+
 	*ppos = ((loff_t) index << PAGE_CACHE_SHIFT) + offset;
 	if (cached_page)
 		page_cache_release(cached_page);

_