linux-kernel - Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max

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Date:   Fri, 23 Sep 2016 00:30:20 +0800
From:   zijun_hu <zijun_hu@...o.com>
To:     Michal Hocko <mhocko@...nel.org>, npiggin@...e.de,
        npiggin@...il.com
Cc:     zijun_hu@....com, David Rientjes <rientjes@...gle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, tj@...nel.org,
        mingo@...nel.org, iamjoonsoo.kim@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max_pages() return value

On 2016/9/22 20:37, Michal Hocko wrote:
> On Thu 22-09-16 09:13:50, zijun_hu wrote:
>> On 09/22/2016 08:35 AM, David Rientjes wrote:
> [...]
>>> 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); 
}