Re: [PATCH] mm: set khugepaged_max_ptes_none by 1/8 of HPAGE_PMD_NR

[Rik van Riel <riel@redhat.com>]

On 02/27/2015 03:53 PM, David Rientjes wrote:
> On Fri, 27 Feb 2015, Ebru Akagunduz wrote:
> 
>> Using THP, programs can access memory faster, by having the
>> kernel collapse small pages into large pages. The parameter
>> max_ptes_none specifies how many extra small pages (that are
>> not already mapped) can be allocated when collapsing a group
>> of small pages into one large page.
>>
> 
> Not exactly, khugepaged isn't "allocating" small pages to collapse into a 
> hugepage, rather it is allocating a hugepage and then remapping the 
> pageblock's mapped pages.

How would you describe the amount of extra memory
allocated, as a result of converting a partially
mapped 2MB area into a THP?

It is not physically allocating 4kB pages, but
I would like to keep the text understandable to
people who do not know the THP internals.

>> A larger value of max_ptes_none can cause the kernel
>> to collapse more incomplete areas into THPs, speeding
>> up memory access at the cost of increased memory use.
>> A smaller value of max_ptes_none will reduce memory
>> waste, at the expense of collapsing fewer areas into
>> THPs.
>>
> 
> This changelog only describes what max_ptes_none does, it doesn't state 
> why you want to change it from HPAGE_PMD_NR-1, which is 511 on x86_64 
> (largest value, more thp), to HPAGE_PMD_NR/8, which is 64 (smaller value, 
> less thp, less rss as a result of collapsing).
>
> This has particular performance implications on users who already have thp 
> enabled, so it's difficult to change the default.  This is tuanble that 
> you could easily set in an initscript, so I don't think we need to change 
> the value for everybody.

I think we do need to change the default.

Why? See this bug:

>> The problem was reported here:
>> https://bugzilla.kernel.org/show_bug.cgi?id=93111

Now, there may be a better value than HPAGE_PMD_NR/8, but
I am not sure what it would be, or why.

I do know that HPAGE_PMD_NR-1 results in undesired behaviour,
as seen in the bug above...

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