On Thu, Jul 25, 2024 at 10:55 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > > Chris Li <chrisl@xxxxxxxxxx> writes: > > > On Thu, Jul 25, 2024 at 7:07 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: > >> > If the freeing of swap entry is random distribution. You need 16 > >> > continuous swap entries free at the same time at aligned 16 base > >> > locations. The total number of order 4 free swap space add up together > >> > is much lower than the order 0 allocatable swap space. > >> > If having one entry free is 50% probability(swapfile half full), then > >> > having 16 swap entries is continually free is (0.5) EXP 16 = 1.5 E-5. > >> > If the swapfile is 80% full, that number drops to 6.5 E -12. > >> > >> This depends on workloads. Quite some workloads will show some degree > >> of spatial locality. For a workload with no spatial locality at all as > >> above, mTHP may be not a good choice at the first place. > > > > The fragmentation comes from the order 0 entry not from the mTHP. mTHP > > have their own valid usage case, and should be separate from how you > > use the order 0 entry. That is why I consider this kind of strategy > > only works on the lucky case. I would much prefer the strategy that > > can guarantee work not depend on luck. > > It seems that you have some perfect solution. Will learn it when you > post it. No, I don't have perfect solutions. I see puting limit on order 0 swap usage and writing out discontinuous swap entries from a folio are more deterministic and not depend on luck. Both have their price to pay as well. > > >> >> - Order-4 pages need to be swapped out, but no enough order-4 non-full > >> >> clusters available. > >> > > >> > Exactly. > >> > > >> >> > >> >> So, we need a way to migrate non-full clusters among orders to adjust to > >> >> the various situations automatically. > >> > > >> > There is no easy way to migrate swap entries to different locations. > >> > That is why I like to have discontiguous swap entries allocation for > >> > mTHP. > >> > >> We suggest to migrate non-full swap clsuters among different lists, not > >> swap entries. > > > > Then you have the down side of reducing the number of total high order > > clusters. By chance it is much easier to fragment the cluster than > > anti-fragment a cluster. The orders of clusters have a natural > > tendency to move down rather than move up, given long enough time of > > random access. It will likely run out of high order clusters in the > > long run if we don't have any separation of orders. > > As my example above, you may have almost 0 high-order clusters forever. > So, your solution only works for very specific use cases. It's not a > general solution. One simple solution is having an optional limitation of 0 order swap. I understand you don't like that option, but there is no other easy solution to achieve the same effectiveness, so far. If there is, I like to hear it. > > >> >> But yes, data is needed for any performance related change. > >> > >> BTW: I think non-full cluster isn't a good name. Partial cluster is > >> much better and follows the same convention as partial slab. > > > > I am not opposed to it. The only reason I hold off on the rename is > > because there are patches from Kairui I am testing depending on it. > > Let's finish up the V5 patch with the swap cache reclaim code path > > then do the renaming as one batch job. We actually have more than one > > list that has the clusters partially full. It helps reduce the repeat > > scan of the cluster that is not full but also not able to allocate > > swap entries for this order. Just the name of one of them as > > "partial" is not precise either. Because the other lists are also > > partially full. We'd better give them precise meaning systematically. > > I don't think that it's hard to do a search/replace before the next > version. The overhead is on the other internal experimental patches. Again, I am not opposed to renaming it. Just want to do it at one batch not many times, including other list names. Chris
