Linux Virtualization - Jan 2021 - [RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO

On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen
wrote:
> On 12/21/20 8:30 AM, Liang Li wrote:
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -137,6 +137,9 @@ enum pageflags {
> >  #endif
> >  #ifdef CONFIG_64BIT
> >  	PG_arch_2,
> > +#endif
> > +#ifdef CONFIG_PREZERO_PAGE
> > +	PG_zero,
> >  #endif
> >  	__NR_PAGEFLAGS,
> 
> I don't think this is worth a generic page->flags bit.
> 
> There's a ton of space in 'struct page' for pages that are in
the
> allocator.  Can't we use some of that space?
I was going to object to that too, but I think the entire approach is
flawed and needs to be thrown out.  It just nukes the caches in extremely
subtle and hard to measure ways, lowering overall system performance.

On Mon, Jan 4, 2021 at 11:28 AM Matthew Wilcox <willy at infradead.org>
wrote:
>
> On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
> > On 12/21/20 8:30 AM, Liang Li wrote:
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -137,6 +137,9 @@ enum pageflags {
> > >  #endif
> > >  #ifdef CONFIG_64BIT
> > >     PG_arch_2,
> > > +#endif
> > > +#ifdef CONFIG_PREZERO_PAGE
> > > +   PG_zero,
> > >  #endif
> > >     __NR_PAGEFLAGS,
> >
> > I don't think this is worth a generic page->flags bit.
> >
> > There's a ton of space in 'struct page' for pages that are
in the
> > allocator.  Can't we use some of that space?
>
> I was going to object to that too, but I think the entire approach is
> flawed and needs to be thrown out.  It just nukes the caches in extremely
> subtle and hard to measure ways, lowering overall system performance.
At a minimum the performance analysis should at least try to quantify
that externalized cost. Certainly that overhead went somewhere. Maybe
if this overhead was limited to run when the CPU would otherwise go
idle, but that might mean it never runs in practice...

On 1/4/21 11:27 AM, Matthew Wilcox wrote:
> On Mon, Jan 04, 2021 at 11:19:13AM -0800, Dave Hansen wrote:
>> On 12/21/20 8:30 AM, Liang Li wrote:
>>> --- a/include/linux/page-flags.h
>>> +++ b/include/linux/page-flags.h
>>> @@ -137,6 +137,9 @@ enum pageflags {
>>>  #endif
>>>  #ifdef CONFIG_64BIT
>>>  	PG_arch_2,
>>> +#endif
>>> +#ifdef CONFIG_PREZERO_PAGE
>>> +	PG_zero,
>>>  #endif
>>>  	__NR_PAGEFLAGS,
>>
>> I don't think this is worth a generic page->flags bit.
>>
>> There's a ton of space in 'struct page' for pages that are
in the
>> allocator.  Can't we use some of that space?
> 
> I was going to object to that too, but I think the entire approach is
> flawed and needs to be thrown out.  It just nukes the caches in extremely
> subtle and hard to measure ways, lowering overall system performance.
Yeah, it certainly can't be the default, but it *is* useful for thing
where we know that there are no cache benefits to zeroing close to where
the memory is allocated.

The trick is opting into it somehow, either in a process or a VMA.