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SubjectRe: [patch] real-time enhanced page allocator and throttling
Robert Love <rml@tech9.net> wrote:
>
> > There is a very good argument for giving !SCHED_OTHER tasks
> > "special treatment" in the VM.
>
> Yes, there is.
>
> Attached patch is against 2.6.0-test2-mm4.

Thanks. I guess we should extend the special treatment in the page
allocator and in balance_dirty_pages() we still should go and update the
various bits of state and poke pdflush if needed.

These changes should yield quite significant improvements in worst-case
latencies for realtime tasks under some situations. I'll test them...


diff -puN mm/page_alloc.c~rt-tasks-special-vm-treatment-2 mm/page_alloc.c
--- 25/mm/page_alloc.c~rt-tasks-special-vm-treatment-2 2003-08-05 16:57:51.000000000 -0700
+++ 25-akpm/mm/page_alloc.c 2003-08-05 16:58:11.000000000 -0700
@@ -592,6 +592,8 @@ __alloc_pages(unsigned int gfp_mask, uns
local_min = z->pages_min;
if (gfp_mask & __GFP_HIGH)
local_min >>= 2;
+ if (rt_task(p))
+ local_min >>= 1;
min += local_min;
if (z->free_pages >= min ||
(!wait && z->free_pages >= z->pages_high)) {
diff -puN mm/page-writeback.c~rt-tasks-special-vm-treatment-2 mm/page-writeback.c
--- 25/mm/page-writeback.c~rt-tasks-special-vm-treatment-2 2003-08-05 16:57:51.000000000 -0700
+++ 25-akpm/mm/page-writeback.c 2003-08-05 17:02:40.000000000 -0700
@@ -144,7 +144,7 @@ get_dirty_limits(struct page_state *ps,
* If we're over `background_thresh' then pdflush is woken to perform some
* writeout.
*/
-void balance_dirty_pages(struct address_space *mapping)
+static void balance_dirty_pages(struct address_space *mapping)
{
struct page_state ps;
long nr_reclaimable;
@@ -167,8 +167,9 @@ void balance_dirty_pages(struct address_
nr_reclaimable = ps.nr_dirty + ps.nr_unstable;
if (nr_reclaimable + ps.nr_writeback <= dirty_thresh)
break;
-
dirty_exceeded = 1;
+ if (rt_task(current))
+ break;

/* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
* Unstable writes are a feature of certain networked
@@ -223,7 +224,7 @@ void balance_dirty_pages_ratelimited(str
* Check the rate limiting. Also, we do not want to throttle real-time
* tasks in balance_dirty_pages(). Period.
*/
- if (get_cpu_var(ratelimits)++ >= ratelimit && !rt_task(current)) {
+ if (get_cpu_var(ratelimits)++ >= ratelimit) {
__get_cpu_var(ratelimits) = 0;
put_cpu_var(ratelimits);
balance_dirty_pages(mapping);
_

-
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