linux-kernel - Re: missing madvise functionality

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Message-Id: <20070405100848.db97d835.dada1@cosmosbay.com>
Date:	Thu, 5 Apr 2007 10:08:48 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Jakub Jelinek <jakub@...hat.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>
Subject: Re: missing madvise functionality

On Thu, 05 Apr 2007 03:31:24 -0400
Rik van Riel <riel@...hat.com> wrote:

> Jakub Jelinek wrote:
> 
> > My guess is that all the page zeroing is pretty expensive as well and
> > takes significant time, but I haven't profiled it.
> 
> With the attached patch (Andrew, I'll change the details around
> if you want - I just wanted something to test now), your test
> case run time went down considerably.
> 
> I modified the test case to only run 1000 loops, so it would run
> a bit faster on my system.  I also modified it to use MADV_DONTNEED
> to zap the pages, instead of the mmap(PROT_NONE) thing you use.
> 

Interesting...

Could you please add this patch and see if it helps on your machine ?

[PATCH] VM : mm_struct's mmap_cache should be close to mmap_sem

Avoids cache line dirtying : The first cache line of mm_struct is/should_be mostly read.

In case find_vma() hits the cache, we dont need to access the begining of mm_struct.
Since we just dirtied mmap_sem, access to its cache line is free.

In case find_vma() misses the cache, we dont need to dirty the begining of mm_struct.


Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

--- linux-2.6.21-rc5/include/linux/sched.h
+++ linux-2.6.21-rc5-ed/include/linux/sched.h
@@ -310,7 +310,6 @@ typedef unsigned long mm_counter_t;
 struct mm_struct {
 	struct vm_area_struct * mmap;		/* list of VMAs */
 	struct rb_root mm_rb;
-	struct vm_area_struct * mmap_cache;	/* last find_vma result */
 	unsigned long (*get_unmapped_area) (struct file *filp,
 				unsigned long addr, unsigned long len,
 				unsigned long pgoff, unsigned long flags);
@@ -324,6 +323,7 @@ struct mm_struct {
 	atomic_t mm_count;			/* How many references to "struct mm_struct" (users count as 1) */
 	int map_count;				/* number of VMAs */
 	struct rw_semaphore mmap_sem;
+	struct vm_area_struct * mmap_cache;	/* last find_vma result */
 	spinlock_t page_table_lock;		/* Protects page tables and some counters */
 
 	struct list_head mmlist;		/* List of maybe swapped mm's.  These are globally strung



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