On Mon, Feb 10, 2025 at 03:55:47PM +0000, Matthew Wilcox wrote: > On Mon, Feb 10, 2025 at 10:47:58AM -0500, Gregory Price wrote: > > I also think someone should actively ask whether `struct page` can be > > hosted on remote memory without performance loss. I may look into this. > > Given that it contains a refcount and various flags, some of which > are quite hot, I would expect performance to suffer. It also suffers > contention between different CPUs, so depending on your cache protocol > (can it do cache-to-cche transfers or does it have to be written back > to memory first?) it may perform quite poorly. But this is something > that can be measured. > > Of course, the question must be asked whetheer we care. Certainly Intel's > Apache Pass and similar Optane RAM products put the memmap on the 3DXP > because there wasn't enough DRAM to put it there. So the pages are > slower, but they were slower anyway! > Well, *if* said memory is intended to host cold(er) data, then we may find the structures to describe those pages aren't particularly hot or contended. This is my suspicion - and I'd rather limit kernel resource allocation on remote memory than try to move kernel resources around. Plus this would still enables hot-unplug. Once all the zone movable regions are clicked off, the page-desc regions are unused... probably. Would just be nice to have some concrete data on when greater zone movable capacity becomes a net-negative. We're making the assumption this this occurs fairly early. ~Gregory
