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SubjectRe: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated values
On Mon, Feb 23, 2009 at 10:23:52AM -0500, Christoph Lameter wrote:
> On Sun, 22 Feb 2009, Mel Gorman wrote:
> > Every page allocation uses gfp_zone() to calcuate what the highest zone
> > allowed by a combination of GFP flags is. This is a large number of branches
> > to have in a fast path. This patch replaces the branches with a lookup
> > table that is calculated at boot-time and stored in the read-mostly section
> > so it can be shared. This requires __GFP_MOVABLE to be redefined but it's
> > debatable as to whether it should be considered a zone modifier or not.
> Are you sure that this is a benefit?

I haven't proved it, but my thinking was that this was a crapload of
branches in fast paths that will be mispredicted. I tried profiling the
RETIRED_MISPREDICTED_BRANCH_INSTRUCTIONS event and noticed there were large
numbers in this general area. However, the profile counters for this event
seem to be very difficult to isolate to a specific area of code so it's
difficult to be 100% certain.

> Jumps are forward and pretty short
> and the compiler is optimizing a branch away in the current code.

I was concerned with mispredictions here rather than the actual assembly
and gfp_zone is inlined so it's lot of branches introduced in a lot of paths.

> 0xffffffff8027bde8 <try_to_free_pages+95>: mov %esi,-0x58(%rbp)
> 0xffffffff8027bdeb <try_to_free_pages+98>: movq $0xffffffff80278cd0,-0x48(%rbp)
> 0xffffffff8027bdf3 <try_to_free_pages+106>: test $0x1,%r8b
> 0xffffffff8027bdf7 <try_to_free_pages+110>: mov 0x620(%rax),%rax
> 0xffffffff8027bdfe <try_to_free_pages+117>: mov %rax,-0x88(%rbp)
> 0xffffffff8027be05 <try_to_free_pages+124>: jne 0xffffffff8027be2c <try_to_free_pages+163>
> 0xffffffff8027be07 <try_to_free_pages+126>: mov $0x1,%r14d
> 0xffffffff8027be0d <try_to_free_pages+132>: test $0x4,%r8b
> 0xffffffff8027be11 <try_to_free_pages+136>: jne 0xffffffff8027be2c <try_to_free_pages+163>
> 0xffffffff8027be13 <try_to_free_pages+138>: xor %r14d,%r14d
> 0xffffffff8027be16 <try_to_free_pages+141>: and $0x100002,%r8d
> 0xffffffff8027be1d <try_to_free_pages+148>: cmp $0x100002,%r8d
> 0xffffffff8027be24 <try_to_free_pages+155>: sete %r14b
> 0xffffffff8027be28 <try_to_free_pages+159>: add $0x2,%r14d
> 0xffffffff8027be2c <try_to_free_pages+163>: mov %gs:0x8,%rdx

Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab

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