On Thu, Feb 25, 2021 at 04:16:33PM +0100, Jesper Dangaard Brouer wrote: > > On Wed, Feb 24, 2021 at 07:56:51PM +0100, Jesper Dangaard Brouer wrote: > > > Avoid multiplication (imul) operations when accessing: > > > zone->free_area[order].nr_free > > > > > > This was really tricky to find. I was puzzled why perf reported that > > > rmqueue_bulk was using 44% of the time in an imul operation: > > > > > > ??? del_page_from_free_list(): > > > 44,54 ??? e2: imul $0x58,%rax,%rax > > > > > > This operation was generated (by compiler) because the struct free_area have > > > size 88 bytes or 0x58 hex. The compiler cannot find a shift operation to use > > > and instead choose to use a more expensive imul, to find the offset into the > > > array free_area[]. > > > > > > The patch align struct free_area to a cache-line, which cause the > > > compiler avoid the imul operation. The imul operation is very fast on > > > modern Intel CPUs. To help fast-path that decrement 'nr_free' move the > > > member 'nr_free' to be first element, which saves one 'add' operation. > > > > > > Looking up instruction latency this exchange a 3-cycle imul with a > > > 1-cycle shl, saving 2-cycles. It does trade some space to do this. > > > > > > Used: gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2) > > > > > > > I'm having some trouble parsing this and matching it to the patch itself. > > > > First off, on my system (x86-64), the size of struct free area is 72, > > not 88 bytes. For either size, cache-aligning the structure is a big > > increase in the struct size. > > Yes, the increase in size is big. For the struct free_area 40 bytes for > my case and 56 bytes for your case. The real problem is that this is > multiplied by 11 (MAX_ORDER) and multiplied by number of zone structs > (is it 5?). Thus, 56*11*5 = 3080 bytes. > > Thus, I'm not sure it is worth it! As I'm only saving 2-cycles, for > something that depends on the compiler generating specific code. And > the compiler can easily change, and "fix" this on-its-own in a later > release, and then we are just wasting memory. > > I did notice this imul happens 45 times in mm/page_alloc.o, with this > offset 0x58, but still this is likely not on hot-path. > Yeah, I'm not convinced it's worth it. The benefit of 2 cycles is small and it's config-dependant. While some configurations will benefit, others do not but the increased consumption is universal. I think there are better ways to save 2 cycles in the page allocator and this seems like a costly micro-optimisation. > > <SNIP> > > > > With gcc-9, I'm also not seeing the imul instruction outputted like you > > described in rmqueue_pcplist which inlines rmqueue_bulk. At the point > > where it calls get_page_from_free_area, it's using shl for the page list > > operation. This might be a compiler glitch but given that free_area is a > > different size, I'm less certain and wonder if something else is going on. > > I think it is the size variation. > Yes. > > Finally, moving nr_free to the end and cache aligning it will make the > > started of each free_list cache-aligned because of its location in the > > struct zone so what purpose does __pad_to_align_free_list serve? > > The purpose of purpose of __pad_to_align_free_list is because struct > list_head is 16 bytes, thus I wanted to align free_list to 16, given we > already have wasted the space. > Ok, that's fair enough but it's also somewhat of a micro-optimisation as whether it helps or not depends on the architecture. I don't think I'll pick this up, certainly in the context of the bulk allocator but it's worth keeping in mind. It's an interesting corner case at least. -- Mel Gorman SUSE Labs
