On Tue, Sep 22, 2020 at 09:50:02AM +0200, Michal Hocko wrote: > [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? That would be 100-300 (maybe more) pages -per- -CPU-, so yes, some people will care quite deeply about this. Thanx, Paul > > 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
