Re: Background page clearing

[Dave Hansen <dave.hansen@intel.com>]

On 07/25/2014 09:27 AM, Wilco Dijkstra wrote:
>> On 07/25/2014 08:06 AM, Wilco Dijkstra wrote:
>>> Is there a reason Linux does not do background page clearing like other OSes to reduce this
>>> overhead? It would be a good fit for typical mobile workloads (bursts of high activity
>> followed by
>>> periods of low activity).
>>
>> If the page is being allocated, it is about to be used and be brought in
>> to the CPU's cache.  If we zero it close to this use, we only pay to
>> bring it in to the CPU's cache once.  Or so goes the theory...
> 
> I can see the reasoning for 4KB pages and small allocations (eg. stack),
> but would that ever be true for huge pages?

Probably not, but huge pages aren't allocated and freed enough in any
workload that I know of for this to make a difference for them.

>> I tried a zero-on-free implementation a year or so ago.  It helped some
>> workloads and hurt others.  The gains were not large enough or
>> widespread enough to merit pushing it in to the kernel.
> 
> Was that literally zero-on-free or zero in the background? Was the result
> the same for different page sizes? My guess is that the result will be
> different for huge pages.

Literally zero-on-free for 4k pages only.  I did it inside the
per-cpu-pages lists.


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