[Cc Mel - the thread starts http://lkml.kernel.org/r/20200918194817.48921-1-urezki@xxxxxxxxx] On Mon 21-09-20 21:48:19, Uladzislau Rezki wrote: > Hello, Michal. > > > > > > > Yes, I do well remember that you are unhappy with this approach. > > > Unfortunately, thus far, there is no solution that makes all developers > > > happy. You might be glad to hear that we are also looking into other > > > solutions, each of which makes some other developers unhappy. So we > > > are at least not picking on you alone. :-/ > > > > No worries I do not feel like a whipping boy here. But do expect me to > > argue against the approach. I would also appreciate it if there was some > > more information on other attempts, why they have failed. E.g. why > > pre-allocation is not an option that works well enough in most > > reasonable workloads. > Pre-allocating has some drawbacks: > > a) It is impossible to predict how many pages will be required to > cover a demand that is controlled by different workloads on > various systems. Yes, this is not trivial but not a rocket science either. Remember that you are relying on a very dumb watermark based pcp pool from the allocator. Mimicing a similar implementation shouldn't be all that hard and you will get your own pool which doesn't affect other page allocator users as much as a bonus. > b) Memory overhead since we do not know how much pages should be > preloaded: 100, 200 or 300 Does anybody who really needs this optimization actually cares about 300 pages? > As for memory overhead, it is important to reduce it because of > embedded devices like phones, where a low memory condition is a > big issue. In that sense pre-allocating is something that we strongly > would like to avoid. How big "machines" are we talking about here? I would expect that really tiny machines would have hard times to really fill up thousands of pages with pointers to free... Would a similar scaling as the page allocator feasible. Really I mostly do care about shared nature of the pcp allocator list that one user can easily monopolize with this API. > > I would also appreciate some more thoughts why we > > need to optimize for heavy abusers of RCU (like close(open) extremes). > > > I think here is a small misunderstanding. Please note, that is not only > about performance and corner cases. There is a single argument support > of the kvfree_rcu(ptr), where maintaining an array in time is needed. > The fallback of the single argument case is extrimely slow. This should be part of the changelog. > > Single-argument details is here: https://lkml.org/lkml/2020/4/28/1626 Error 501 > > > > I strongly agree with Thomas http://lkml.kernel.org/r/87tux4kefm.fsf@xxxxxxxxxxxxxxxxxxxxxxx > > > > that this optimization is not aiming at reasonable workloads. Really, go > > > > with pre-allocated buffer and fallback to whatever slow path you have > > > > already. Exposing more internals of the allocator is not going to do any > > > > good for long term maintainability. > > > > > > I suggest that you carefully re-read the thread following that email. > > > > I clearly remember Thomas not being particularly happy that you optimize > > for a corner case. I do not remember there being a consensus that this > > is the right approach. There was some consensus that this is better than > > a gfp flag. Still quite bad though if you ask me. > > > > > Given a choice between making users unhappy and making developers > > > unhappy, I will side with the users each and every time. > > > > Well, let me rephrase. It is not only about me (as a developer) being > > unhappy but also all the side effects this would have for users when > > performance of their favorite workload declines for no apparent reason > > just because pcp caches are depleted by an unrelated process. > > > If depleted, we have a special worker that charge it. From the other hand, > the pcplist can be depleted by its nature, what _is_ not wrong. But just > in case we secure it since you had a concern about it. pcp free lists should ever get empty when we run out of memory and need to reclaim. Otherwise they are constantly refilled/rebalanced on demand. The fact that you are refilling them from outside just suggest that you are operating on a wrong layer. Really, create your own pool of pages and rebalance them based on the workload. > Could you please specify a real test case or workload you are talking about? I am not a performance expert but essentially any memory allocator heavy workload might notice. I am pretty sure Mel would tell you more. -- Michal Hocko SUSE Labs
