On Mon, Sep 02, 2024 at 02:53:26PM +0800, Huang, Ying wrote: > Gregory Price <gourry@xxxxxxxxxx> writes: > > > On Mon, Aug 19, 2024 at 03:46:00PM +0800, Huang, Ying wrote: > >> Gregory Price <gourry@xxxxxxxxxx> writes: > >> > >> > Unmapped pagecache pages can be demoted to low-tier memory, but > >> > they can only be promoted if a process maps the pages into the > >> > memory space (so that NUMA hint faults can be caught). This can > >> > cause significant performance degradation as the pagecache ages > >> > and unmapped, cached files are accessed. > >> > > >> > This patch series enables the pagecache to request a promotion of > >> > a folio when it is accessed via the pagecache. > >> > > >> > We add a new `numa_hint_page_cache` counter in vmstat to capture > >> > information on when these migrations occur. > >> > >> It appears that you will promote page cache page on the second access. > >> Do you have some better way to identify hot pages from the not-so-hot > >> pages? How to balance between unmapped and mapped pages? We have hot > >> page selection for hot pages. > >> > >> [snip] > >> > > > > I've since explored moving this down under a (referenced && active) check. > > > > This would be more like promotion on third access within an LRU shrink > > round (the LRU should, in theory, hack off the active bits on some decent > > time interval when the system is pressured). > > > > Barring adding new counters to folios to track hits, I don't see a clear > > and obvious way way to track hotness. The primary observation here is > > that pagecache is un-mapped, and so cannot use numa-fault hints. > > > > This is more complicated with MGLRU, but I'm saving that for after I > > figure out the plan for plain old LRU. > > Several years ago, we have tried to use the access time tracking > mechanism of NUMA balancing to track the access time latency of unmapped > file cache folios. The original implementation is as follows, > > https://git.kernel.org/pub/scm/linux/kernel/git/vishal/tiering.git/commit/?h=tiering-0.8&id=5f2e64ce75c0322602c2ec8c70b64bb69b1f1329 > > What do you think about this? > Also seems like an interesting option. I've been looking at another old proposal to simply add a new LRU that was implemented by kbusch a few years back. https://git.kernel.org/pub/scm/linux/kernel/git/kbusch/linux.git/commit/?h=lru-promote&id=6616afe9a722f6ebedbb27ade3848cf07b9a3af7 I may spend a little time to add a few different methods in with a switch I can flip to test them side by side / with each other and see what results we can get. > -- > Best Regards, > Huang, Ying
