On 2016/9/22 20:37, Michal Hocko wrote:

On Thu 22-09-16 09:13:50, zijun_hu wrote:

quoted

On 09/22/2016 08:35 AM, David Rientjes wrote:

[...]

quoted

quoted

The intent is as it is implemented; with your change, lazy_max_pages() is 
potentially increased depending on the number of online cpus.  This is 
only a heuristic, changing it would need justification on why the new
value is better.  It is opposite to what the comment says: "to be 
conservative and not introduce a big latency on huge systems, so go with
a less aggressive log scale."  NACK to the patch.

my change potentially make lazy_max_pages() decreased not increased, i seems
conform with the comment

if the number of online CPUs is not power of 2, both have no any difference
otherwise, my change remain power of 2 value, and the original code rounds up
to next power of 2 value, for instance

my change : (32, 64] -> 64
	     32 -> 32, 64 -> 64
the original code: [32, 63) -> 64
                   32 -> 64, 64 -> 128

You still completely failed to explain _why_ this is an improvement/fix
or why it matters. This all should be in the changelog.

Hi npiggin,
could you give some comments for this patch since lazy_max_pages() is introduced
by you

my patch is based on the difference between fls() and get_count_order() mainly
the difference between fls() and get_count_order() will be shown below
more MM experts maybe help to decide which is more suitable

if parameter > 1, both have different return value only when parameter is
power of two, for example

fls(32) = 6 VS get_count_order(32) = 5
fls(33) = 6 VS get_count_order(33) = 6
fls(63) = 6 VS get_count_order(63) = 6
fls(64) = 7 VS get_count_order(64) = 6

@@ -594,7 +594,9 @@ static unsigned long lazy_max_pages(void) 

{ 
    unsigned int log; 

-    log = fls(num_online_cpus()); 
+    log = num_online_cpus(); 
+    if (log > 1) 
+        log = (unsigned int)get_count_order(log); 

    return log * (32UL * 1024 * 1024 / PAGE_SIZE); 
} 

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