Matthew Wilcox
2021-Jan-04 19:27 UTC
[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.
Dan Williams
2021-Jan-04 19:44 UTC
[RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO
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...
Dave Hansen
2021-Jan-04 19:51 UTC
[RFC v2 PATCH 4/4] mm: pre zero out free pages to speed up page allocation for __GFP_ZERO
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.