Re: [PATCH 1/3] readahead: drop behind

[Peter Zijlstra]

On Sat, 2007-07-21 at 16:29 -0400, Eric St-Laurent wrote:
> On Sat, 2007-21-07 at 23:00 +0200, Peter Zijlstra wrote:
> > plain text document attachment (readahead-useonce.patch)
> > Use the read-ahead code to provide hints to page reclaim.
> > 
> > This patch has the potential to solve the streaming-IO trashes my
> > desktop problem.
> > 
> > It tries to aggressively reclaim pages that were loaded in a strong
> > sequential pattern and have been consumed. Thereby limiting the damage
> > to the current resident set.
> > 
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> 
> With the fadvise change, it looks like the right solution to me.
> 
> The patches are for which kernel? They doesn't apply cleanly to
> 2.6.22.1.

They are against git of a few hours ago and the latest readahead patches
from Wu (which don't apply cleanly either, but the rejects are trivial).

> It would be useful to have a temporary /proc tunable to enable/disable
> the heuristic to help test the effects.

Right, I had such a patch somewhere,.. won't apply cleanly but should be
obvious..

---
 kernel/sysctl.c |    9 +++++++++
 mm/readahead.c  |    6 ++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

Index: linux-2.6/kernel/sysctl.c
===================================================================
--- linux-2.6.orig/kernel/sysctl.c
+++ linux-2.6/kernel/sysctl.c
@@ -168,6 +168,7 @@ extern int dirty_ratio_handler(ctl_table
 
 extern int prove_locking;
 extern int lock_stat;
+extern int sysctl_dropbehind;
 
 /* The default sysctl tables: */
 
@@ -1075,6 +1076,14 @@ static ctl_table vm_table[] = {
 		.extra1		= &zero,
 	},
 #endif
+	{
+		.ctl_name	= CTL_UNNUMBERED,
+		.procname	= "drop_behind",
+		.data		= &sysctl_dropbehind,
+		.maxlen		= sizeof(sysctl_dropbehind),
+		.mode		= 0644,
+		.proc_handler	= &proc_dointvec,
+	},
 /*
  * NOTE: do not add new entries to this table unless you have read
  * Documentation/sysctl/ctl_unnumbered.txt
Index: linux-2.6/mm/readahead.c
===================================================================
--- linux-2.6.orig/mm/readahead.c
+++ linux-2.6/mm/readahead.c
@@ -429,6 +429,8 @@ void page_cache_sync_readahead(struct ad
 }
 EXPORT_SYMBOL_GPL(page_cache_sync_readahead);
 
+int sysctl_dropbehind = 2;
+
 /**
  * page_cache_async_readahead - file readahead for marked pages
  * @mapping: address_space which holds the pagecache and I/O vectors
@@ -473,7 +475,7 @@ page_cache_async_readahead(struct addres
 	 * hint that there is sequential IO, which implies that the pages that
 	 * have been used thus far can be reclaimed
 	 */
-	if (ra->size == ra->ra_pages) {
+	if (sysctl_dropbehind && ra->size == ra->ra_pages) {
 		unsigned long demote_idx = offset - min(offset, ra->size);
 
 		read_lock_irq(&mapping->tree_lock);
@@ -489,7 +491,7 @@ page_cache_async_readahead(struct addres
 			 * users. We should not take away somebody else's
 			 * pages, so do not drop behind beyond this point.
 			 */
-			if (PageActive(page))
+			if (sysctl_dropbehind > 1 && PageActive(page))
 				break;
 
 			lru_demote(page);


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