Re: [RFC v3 PATCH 0/5] Promotion of Unmapped Page Cache Folios.

[Gregory Price <gourry@gourry.net>]

On Thu, Jan 23, 2025 at 11:46:49AM +0800, Huang, Ying wrote:
> Gregory Price <gourry@gourry.net> writes:
> > Test 2 shows overhead of TPP on + pagecache promo off
> > Test 3 shows overhead of TPP+Promo on, but all the memory is on top tier
> >
> > This shows the check as to whether the folio is in the top tier is
> > actually somewhat expensive (~5% compared to baseline, ~2.7% compared to
> > TPP-on Promo-off).
> 
> This is unexpected.  Can we try to optimize it?  For example, via using
> a nodemask?  node_is_toptier() is used in the mapped pages promotion
> too (1 vs. 2 above).  I guess that the optimization can reduce the
> overhead there with measurable difference too.
>

Agreed it surprised me a bit as well. But more surprising is the fact
that test 2 was also 2-3% slower given that it's a simple boolean check
against whether tiering is turned on.

I suppose that since the test is blowing up the cache/tlb by design,
multiple additional cache/tlb misses could cause a non-trivial slowdown,
but it is certainly a small puzzle I haven't dug into yet.

~Gregory