On Thu, Feb 25, 2021 at 04:16:33PM +0100, Jesper Dangaard Brouer wrote:

quoted

On Wed, Feb 24, 2021 at 07:56:51PM +0100, Jesper Dangaard Brouer wrote:

quoted

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.

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<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.

quoted

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