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

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Message-ID: <20160923133022.47cfd3dd@roar.ozlabs.ibm.com>
Date:   Fri, 23 Sep 2016 13:30:22 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     zijun_hu <zijun_hu@...o.com>
Cc:     Michal Hocko <mhocko@...nel.org>, npiggin@...e.de,
        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 Fri, 23 Sep 2016 00:30:20 +0800
zijun_hu <zijun_hu@...o.com> wrote:

> 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); 
> } 
> 

To be honest, I don't think I chose it with a lot of analysis.
It will depend on the kernel usage patterns, the arch code,
and the CPU microarchitecture, all of which would have changed
significantly.

I wouldn't bother changing it unless you do some benchmarking
on different system sizes to see where the best performance is.
(If performance is equal, fewer lazy pages would be better.)

Good to see you taking a look at this vmalloc stuff. Don't be
discouraged if you run into some dead ends.

Thanks,
Nick