On Mon, Apr 18, 2022 at 3:58 AM Barry Song <21cnbao@xxxxxxxxx> wrote: > > On Thu, Apr 7, 2022 at 3:16 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > > > To avoid confusion, the terms "promotion" and "demotion" will be > > applied to the multi-gen LRU, as a new convention; the terms > > "activation" and "deactivation" will be applied to the active/inactive > > LRU, as usual. > > > > The aging produces young generations. Given an lruvec, it increments > > max_seq when max_seq-min_seq+1 approaches MIN_NR_GENS. The aging > > promotes hot pages to the youngest generation when it finds them > > accessed through page tables; the demotion of cold pages happens > > consequently when it increments max_seq. The aging has the complexity > > O(nr_hot_pages), since it is only interested in hot pages. Promotion > > in the aging path does not require any LRU list operations, only the > > updates of the gen counter and lrugen->nr_pages[]; demotion, unless as > > the result of the increment of max_seq, requires LRU list operations, > > e.g., lru_deactivate_fn(). > > > > The eviction consumes old generations. Given an lruvec, it increments > > min_seq when the lists indexed by min_seq%MAX_NR_GENS become empty. A > > feedback loop modeled after the PID controller monitors refaults over > > anon and file types and decides which type to evict when both types > > are available from the same generation. > > > > Each generation is divided into multiple tiers. Tiers represent > > different ranges of numbers of accesses through file descriptors. A > > page accessed N times through file descriptors is in tier > > order_base_2(N). Tiers do not have dedicated lrugen->lists[], only > > bits in folio->flags. In contrast to moving across generations, which > > requires the LRU lock, moving across tiers only involves operations on > > folio->flags. The feedback loop also monitors refaults over all tiers > > and decides when to protect pages in which tiers (N>1), using the > > first tier (N=0,1) as a baseline. The first tier contains single-use > > unmapped clean pages, which are most likely the best choices. The > > eviction moves a page to the next generation, i.e., min_seq+1, if the > > feedback loop decides so. This approach has the following advantages: > > 1. It removes the cost of activation in the buffered access path by > > inferring whether pages accessed multiple times through file > > descriptors are statistically hot and thus worth protecting in the > > eviction path. > > 2. It takes pages accessed through page tables into account and avoids > > overprotecting pages accessed multiple times through file > > descriptors. (Pages accessed through page tables are in the first > > tier, since N=0.) > > 3. More tiers provide better protection for pages accessed more than > > twice through file descriptors, when under heavy buffered I/O > > workloads. > > > > Hi Yu, > As I told you before, I tried to change the current LRU (not MGLRU) by only > promoting unmapped file pages to the head of the inactive head rather than > the active head on its second access: > https://lore.kernel.org/lkml/CAGsJ_4y=TkCGoWWtWSAptW4RDFUEBeYXwfwu=fUFvV4Sa4VA4A@xxxxxxxxxxxxxx/ > I have already seen some very good results by the decease of cpu consumption of > kswapd and direct reclamation in the testing. Glad to hear. I suspected you'd see some good results with that change :) > in mglru, it seems "twice" isn't a concern at all, one unmapped file > page accessed > twice has no much difference with those ones which are accessed once as you > only begin to increase refs from the third time: refs are *additional* accesses: PG_referenced: N=1 PG_referenced+PG_workingset: N=2 PG_referenced+PG_workingset+refs: N=3,4,5 When N=2, order_base_2(N)=1. So pages accessed twice are in the second tier. Therefore they are "different". More details [1]: +/* + * Each generation is divided into multiple tiers. Tiers represent different + * ranges of numbers of accesses through file descriptors. A page accessed N + * times through file descriptors is in tier order_base_2(N). A page in the + * first tier (N=0,1) is marked by PG_referenced unless it was faulted in + * though page tables or read ahead. A page in any other tier (N>1) is marked + * by PG_referenced and PG_workingset. + * + * In contrast to moving across generations which requires the LRU lock, moving + * across tiers only requires operations on folio->flags and therefore has a + * negligible cost in the buffered access path. In the eviction path, + * comparisons of refaulted/(evicted+protected) from the first tier and the + * rest infer whether pages accessed multiple times through file descriptors + * are statistically hot and thus worth protecting. + * + * MAX_NR_TIERS is set to 4 so that the multi-gen LRU can support twice of the + * categories of the active/inactive LRU when keeping track of accesses through + * file descriptors. It requires MAX_NR_TIERS-2 additional bits in folio->flags. + */ +#define MAX_NR_TIERS 4U [1] https://lore.kernel.org/linux-mm/20220407031525.2368067-7-yuzhao@xxxxxxxxxx/ > +static void folio_inc_refs(struct folio *folio) > +{ > + unsigned long refs; > + unsigned long old_flags, new_flags; > + > + if (folio_test_unevictable(folio)) > + return; > + > + /* see the comment on MAX_NR_TIERS */ > + do { > + new_flags = old_flags = READ_ONCE(folio->flags); > + > + if (!(new_flags & BIT(PG_referenced))) { > + new_flags |= BIT(PG_referenced); > + continue; > + } > + > + if (!(new_flags & BIT(PG_workingset))) { > + new_flags |= BIT(PG_workingset); > + continue; > + } > + > + refs = new_flags & LRU_REFS_MASK; > + refs = min(refs + BIT(LRU_REFS_PGOFF), LRU_REFS_MASK); > + > + new_flags &= ~LRU_REFS_MASK; > + new_flags |= refs; > + } while (new_flags != old_flags && > + cmpxchg(&folio->flags, old_flags, new_flags) != old_flags); > +} > > So my question is what makes you so confident that twice doesn't need > any special treatment while the vanilla kernel is upgrading this kind of page > to the head of the active instead? I am asking this because I am considering > reclaiming unmapped file pages which are only accessed twice when they > get to the tail of the inactive list. Per above, pages accessed twice are in their own tier. Hope this clarifies it.
