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SubjectRe: [patch] mm, vmacache: hash addresses based on pmd
On Mon, 9 Jul 2018, Andrew Morton wrote:

> > When perf profiling a wide variety of different workloads, it was found
> > that vmacache_find() had higher than expected cost: up to 0.08% of cpu
> > utilization in some cases. This was found to rival other core VM
> > functions such as alloc_pages_vma() with thp enabled and default
> > mempolicy, and the conditionals in __get_vma_policy().
> >
> > VMACACHE_HASH() determines which of the four per-task_struct slots a vma
> > is cached for a particular address. This currently depends on the pfn,
> > so pfn 5212 occupies a different vmacache slot than its neighboring
> > pfn 5213.
> >
> > vmacache_find() iterates through all four of current's vmacache slots
> > when looking up an address. Hashing based on pfn, an address has
> > ~1/VMACACHE_SIZE chance of being cached in the first vmacache slot, or
> > about 25%, *if* the vma is cached.
> >
> > This patch hashes an address by its pmd instead of pte to optimize for
> > workloads with good spatial locality. This results in a higher
> > probability of vmas being cached in the first slot that is checked:
> > normally ~70% on the same workloads instead of 25%.
>
> Was the improvement quantifiable?
>

I've run page fault testing to answer this question on Haswell since the
initial profiling was done over a wide variety of user-controlled
workloads and there's no guarantee that such profiling would be a fair
comparison either way. For page faulting it's either falling below our
testing levels of 0.02%, or is right at 0.02%. Running without the patch
it's 0.05-0.06% overhead.

> Surprised. That little array will all be in CPU cache and that loop
> should execute pretty quickly? If it's *that* sensitive then let's zap
> the no-longer-needed WARN_ON. And we could hide all the event counting
> behind some developer-only ifdef.
>

Those vmevents are only defined for CONFIG_DEBUG_VM_VMACACHE, so no change
needed there. The WARN_ON() could be moved under the same config option.
I assume that if such a config option exists that at least somebody is
interested in debugging mm/vmacache.c once in a while.

> Did you consider LRU-sorting the array instead?
>

It adds 40 bytes to struct task_struct, but I'm not sure the least
recently used is the first preferred check. If I do
madvise(MADV_DONTNEED) from a malloc implementation where I don't control
what is free()'d and I'm constantly freeing back to the same hugepages,
for example, I may always get first slot cache hits with this patch as
opposed to the 25% chance that the current implementation has (and perhaps
an lru would as well).

I'm sure that I could construct a workload where LRU would be better and
could show that the added footprint were worthwhile, but I could also
construct a workload where the current implementation based on pfn would
outperform all of these. It simply turns out that on the user-controlled
workloads that I was profiling that hashing based on pmd was the win.

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