An Intel Engineer Has Another Optimization For Possible Performance Degradation On Linux
The latest Linux performance work to talk about is Kirill Shutemov having posted a patch on Thursday to allow restoring large pages after fragmentation. This direct mapping fragmentation can degrade performance over time.
Shutemov explained with the "request for comments" patch:
Change of attributes of the pages may lead to fragmentation of direct mapping over time and performance degradation as result.
With current code it's one way road: kernel tries to avoid splitting large pages, but it doesn't restore them back even if page attributes got compatible again.
Any change to the mapping may potentially allow to restore large page.
With the initial patch, no numbers were provided as to the expected performance benefits. Dave Hansen, also of Intel, commented that "Looks pretty sane to me, and sounds like something we've needed for a long time."
We'll see if this optimization makes it for Linux 5.8.