On Wed, Jan 11, 2023 at 6:17 AM Michal Koutný <mkoutny@xxxxxxxx> wrote: > > On Wed, Dec 14, 2022 at 02:51:21PM -0800, Yuanchu Xie <yuanchu@xxxxxxxxxx> wrote: > > that's frequently used. The only missing pieces between MGLRU > > generations and working set estimation are a consistent aging cadence > > and an interface; we introduce the two additions. > > > > Periodic aging > > ====== > > MGLRU Aging is currently driven by reclaim, so the amount of time > > between generations is non-deterministic. With memcgs being aged > > regularly, MGLRU generations become time-based working set information. > > Is this periodic aging specific to memcgs? IOW, periodic aging isn't > needed without memcgs (~with root only) > (Perhaps similar question to Aneeh's.) Originally, I didn't see much value in periodic aging without memcgs, as the main goal was to provide working set information. Periodic aging might lead to MGLRU making better reclaim decisions, but I don't have any benchmarks to back it up right now. > > > Use case: proactive reclaimer > > ====== > > The proactive reclaimer sets the aging interval, and periodically reads > > the page idle age stats, forming a working set estimation, which it then > > calculates an amount to write to memory.reclaim. > > > > With the page idle age stats, a proactive reclaimer could calculate a > > precise amount of memory to reclaim without continuously probing and > > inducing reclaim. > > Could the aging be also made per-memcg? (Similar to memory.reclaim, > possibly without the new kthread (if global reclaim's aging is enough).) It is possible. We can have hierarchical aging, invoked by writing to memory.aging with a time duration. For every child memcg, if its young generation is older than (current time - specified duration), do aging. However, now we need a userspace tool to drive the aging, invoking this interface every few seconds, since every memcg is aged at a different cadence. Having a kthread perform aging has the benefit of simplicity, gives a source of truth for the aging interval, and makes the feature more accessible. The application developers, if they want to take a look at the page idle age stats, could do so without needing additional ceremony. Thanks, Yuanchu
