Re: missing madvise functionality

Eric Dumazet
2007-04-05T08:08:48Z

On Thu, 05 Apr 2007 03:31:24 -0400
Rik van Riel <riel@redhat.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@cosmosbay.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