On 08.03.24 08:49, Lance Yang wrote:
The patch reduces the process visible downtime during hugepage
collapse. This is achieved by pre-zeroing the hugepage before
acquiring mmap_lock(write mode) if nr_pte_none >= 256, without
affecting the efficiency of khugepaged.
On an Intel Core i5 CPU, the process visible downtime during
hugepage collapse is as follows:
| nr_ptes_none | w/o __GFP_ZERO | w/ __GFP_ZERO | Change |
--------------------------------------------------—----------
| 511 | 233us | 95us | -59.21%|
| 384 | 376us | 219us | -41.20%|
| 256 | 421us | 323us | -23.28%|
| 128 | 523us | 507us | -3.06%|
Of course, alloc_charge_hpage() will take longer to run with
the __GFP_ZERO flag.
| Func | w/o __GFP_ZERO | w/ __GFP_ZERO |
|----------------------|----------------|---------------|
| alloc_charge_hpage | 198us | 295us |
But it's not a big deal because it doesn't impact the total
time spent by khugepaged in collapsing a hugepage. In fact,
it would decrease.
It does look sane to me and not overly complicated.
But, it's an optimization really only when we have quite a bunch of
pte_none(), possibly repeatedly so that it really makes a difference.
Usually, when we repeatedly collapse that many pte_none() we're just
wasting a lot of memory and should re-evaluate life choices :)
So my question is: do we really care about it that much that we care to
optimize?
But again, LGTM.
--
Cheers,
David / dhildenb
