linux-kernel - Re: [PATCH] mm/page_alloc: increase default min_free

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Date:   Thu, 27 Feb 2020 13:45:57 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>,
        Joel Savitz <jsavitz@...hat.com>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org
Subject: Re: [PATCH] mm/page_alloc: increase default min_free_kbytes bound

On 2/21/20 2:53 AM, John Hubbard wrote:
> On 2/20/20 7:01 AM, Joel Savitz wrote:
>> 
>> Currently, the vm.min_free_kbytes sysctl value is capped at a hardcoded
>> 64M in init_per_zone_wmark_min (unless it is overridden by khugepaged
>> initialization).
>> 
>> This value has not been modified since 2005, and enterprise-grade
>> systems now frequently have hundreds of GB of RAM and multiple 10, 40,
>> or even 100 GB NICs. We have seen page allocation failures on heavily
>> loaded systems related to NIC drivers. These issues were resolved by an
>> increase to vm.min_free_kbytes.
>> 
>> This patch increases the hardcoded value by a factor of 4 as a temporary
>> solution.
>> 
>> Further work to make the calculation of vm.min_free_kbytes more
>> consistent throughout the kernel would be desirable.
>> 
>> As an example of the inconsistency of the current method, this value is
>> recalculated by init_per_zone_wmark_min() in the case of memory hotplug
>> which will override the value set by set_recommended_min_free_kbytes()
>> called during khugepaged initialization even if khugepaged remains
>> enabled, however an on/off toggle of khugepaged will then recalculate
>> and set the value via set_recommended_min_free_kbytes().
>> 
>> Signed-off-by: Joel Savitz <jsavitz@...hat.com>
>> ---
>>  mm/page_alloc.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 3c4eb750a199..32cbfb13e958 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -7867,8 +7867,8 @@ int __meminit init_per_zone_wmark_min(void)
>>  		min_free_kbytes = new_min_free_kbytes;
>>  		if (min_free_kbytes < 128)
>>  			min_free_kbytes = 128;
>> -		if (min_free_kbytes > 65536)
>> -			min_free_kbytes = 65536;
>> +		if (min_free_kbytes > 262144)
>> +			min_free_kbytes = 262144;
> 
> 
> Would it be any better to at least use symbols, instead of numbers, in the
> routine? Like this:

+1

Thanks