On Wed, Jan 17, 2024 at 4:18 PM Chris Li <chrisl@xxxxxxxxxx> wrote: > > Hi Yosry, > > On Wed, Jan 17, 2024 at 3:48 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > > Currently the xarray patch should have everything it takes to use RCU > > > read lock. However taking out the tree spinlock is more work than > > > previously. If we are going to remove the tree spinlock, I think we > > > should revert back to doing a zswap tree lookup and return the zswap > > > entry with reference increased. The tree mapping can still decouple > > > from the zswap entry reference count drop to zero. Anyway, my V1 of > > > the xarray patch will not include removing the tree spinlock. > > > > Interesting. What do you mean by removing the tree spinlock? My > > assumption was that the xarray reduces lock contention because we do > > not need a lock to do lookups, but we still need the lock otherwise. > > Did you have something in mind to completely remove the tree lock? > > In my current xarray series, it adds the xarray alongside the rb tree. > Xarray has its own internal lock as well. Effectively zswap now has > two locks instead of just one previously. The xarray lock will not > have any contention due to the xarray lock taken inside the zswap rb > tree lock. The eventual goal is reducing the two locks back to > one(xarray lock), which is not in my V1 patch. Your understanding is > correct, the xarray still needs to have one lock for protecting the > write path. Hmm I don't understand. What's the point of keeping the rbtree if we have the xarray? Doesn't it end up being more expensive and bug-prone to maintain both trees? When you say "eventual goal", do you mean what the patch would morph into in later versions (as in v1 is just a proof of concept without removing the rbtree), or follow up patches?
