Re: Dirty/Access bits vs. page content

["H. Peter Anvin" <hpa@zytor.com>]

On 04/24/2014 07:46 PM, Linus Torvalds wrote:
> On Thu, Apr 24, 2014 at 7:41 PM, Benjamin Herrenschmidt
> <benh@kernel.crashing.org> wrote:
>> On Fri, 2014-04-25 at 11:37 +1000, Benjamin Herrenschmidt wrote:
>>>
>>> The flip side is that we do a lot more IPIs for large invalidates,
>>> since we drop the PTL on every page table page.
>>
>> Oh I missed that your patch was smart enough to only do that in the
>> presence of non-anonymous dirty pages. That should take care of the
>> common case of short lived programs, those should still fit in a
>> single big batch.
> 
> Right. It only causes extra TLB shootdowns for dirty shared mappings.
> 
> Which, let's face it, don't actually ever happen. Using mmap to write
> to files is actually very rare, because it really sucks in just about
> all possible ways. There are almost no loads where it's not better to
> just use a "write()" system call instead.
> 
> So the dirty shared mapping case _exists_, but it's pretty darn
> unusual. The case where you have lots of mmap/munmap activity is even
> less unusual. I suspect the most common case is for stress-testing the
> VM, because nobody sane does it for an actual real load.
> 

The cases where they occur the mappings tend to be highly stable, i.e.
map once *specifically* to be able to do a whole bunch of things without
system calls, and then unmap when done.

	-hpa


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