On Tue, Apr 19, 2022 at 12:54 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > 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. Yep, I found the trick here , "+1" is magic behind the code, haha. +static int folio_lru_tier(struct folio *folio) +{ + int refs; + unsigned long flags = READ_ONCE(folio->flags); + + refs = (flags & LRU_REFS_FLAGS) == LRU_REFS_FLAGS ? + ((flags & LRU_REFS_MASK) >> LRU_REFS_PGOFF) + 1 : 0; + + return lru_tier_from_refs(refs); +} + TBH, this might need some comments, otherwise, it is easy to misunderstand we are beginning to have protection from 3rd access :-) Thanks barry
