On Thu, May 16, 2024 at 12:23 AM Luis Chamberlain <mcgrof@xxxxxxxxxx> wrote: > > On Wed, May 15, 2024 at 11:15:58PM -0600, Yu Zhao wrote: > > On Wed, May 15, 2024 at 1:34 PM Luis Chamberlain <mcgrof@xxxxxxxxxx> wrote: > > > > > > RFC to see if we have a breakout session today at LSFMM. > > > > > > After the TAO talk today it occurred to me that it might make sense > > > to review how we're measuring memory fragmentation today. We're looking > > > to add automation support into kdevops for this to help compare and > > > contrast memory fragmentation behaviour with one kernel against another. > > > A while ago, while mTHP was being evaluated I asked genearlly how we > > > could measure fragmentation with a simple one value, and John Hubbard > > > had one recommendation [0], working that proved we could simplify things > > > [1] but we also could just use the existing fragmentation index and only > > > consider the values where this is concerned for fragmentation and not > > > lack of memory. It begs the question of how folks are measuring memory > > > fragmentation today in production, and if they have any desirable > > > changes. The first approach being considered is to reproduce the > > > workloads Mel Gorman had written and used for mmtests and leverage those > > > on kdevops, perhaps modernize them, but before we do so it seems > > > reviewing how we measure fragmentation today might be useful to others > > > too. > > > > > > As for mmtests integration into kdevops, first order of business are > > > just a few distro-friendly updates [2], for the next steps after that > > > though it would be great to review the above. > > > > > > [0] https://lore.kernel.org/all/5ac6a387-0ca7-45ca-bebc-c3bdd48452cb@xxxxxxxxxx/T/#u > > > [1] https://lkml.kernel.org/r/20240314005710.2964798-1-mcgrof@xxxxxxxxxx > > > [2] https://lore.kernel.org/kdevops/20240319044621.2682968-1-mcgrof@xxxxxxxxxx/ > > > > Please correct me if I'm wrong -- I don't think we can use a single > > measure to describe fragmentation in an actionable way. > ^^^^^^^^^^ ^^^ > Two key words: actionable way. > > Even in that sense, to say that you need more would suggest that either > compaction does not suffice to address memory fragmentation, or that we > can improve memory fragmentation through other means. Both are possible, > and only measurements can prove that. > > But my point was not about taking measures in an *actionable way* to > address memory fragmentation though, but simply measuring memory > fragmentation in environment A and evironment B, to address the > question, under which environment is memory fragmentation worse. That > said, I am *also* interested in solutions to address memory > fragmentation, but that's a secondary step, first I'd like to measure, > not take action. > > It does not mean that evaluating measurements to consider memory > fragmentation to evaluate actionable measures are not useful to the > single snapshot of memory fragmentation. In fact if more information is > better, or we're lacking other sources of measurement of memory > fragmentation it'd be great to improve it. > > As noted in the above URL John Hubbard provided a simple metric > recommendation, and I tried to implement it but as the patch in [1] > notes the missing semantic would be used folios per order and to add > this I thought it would be expensive today from, as per my last review > (perhaps I am wrong). Hence my approach to only seek one value and see > if its positive, and if so how high. Thanks. IIUC, the metric(s) you have in mind would be able to compare over time or across different systems. I still don't think a single measurement can do that because different orders are not on the same footing (or so to speak), unless we are only interested in one non-zero order. For example, if we have two systems, one has lower fragmentation for some orders but higher fragmentation for the rest, and the other is the opposite. How would we be able to use a single measure to describe this? IOW, I don't think a single measurement can describe all orders in a comparable way, which would be the weakest requirement we would have to impose. > > IMO, we would need at least multiple values, e.g., fragmentation index > > for each non-zero order, to describe how fragmented the memory is with > > respect to the order of interest. > > Here you seem to accept you can measure how memory is fragmented with the > existing fragmentation index for each order, is that right? Correct. Fragmentation indices for all orders are what we have now. > Or is it that this is the only tool we have today, but likely we could > improve the metric? With them we can compare fragmentation in a system over time, or fragmentation between systems. In addition to comparison, we also can tell whether reclaim+compaction would be able to make an allocation of a specific order possible, as I mentioned earlier. > > Of course we could encode multiple > > fragmentation indices into a single value, but that's not really one > > measure. > > If I am not looking for an actionable measure, but just get a single > quantifiable metric of "how badly fragmented is this system", is > a single value not useful for that purpose? As I (badly) explained earlier, a single value can't do that because different orders are not on the same footing (or so to speak), unless we are only interested in one non-zero order. So we would need fragmentation_index[NR_non_zero_orders]. > For my purpose, it was about evaluating if the general situation is > worse in environment A Vs B, in that world, would a single metric work? No, for example, A can allocate 4 order-1 but 0 order-2, and B can allocate 2 order-1 *or* 1 order-2, which one would you say is better or worse? This, IMO, depends on which order you are trying to allocate. Does it make sense? > > Fragmentation index of an order can tell whether reclaim+compaction > > can theoretically result in a free area of that order. > > Indeed, for my interest it's the positive values, about when a system > has memory fragmented. > > > As an average, > > fragmentation index can't tell which actionable unit area, > ^^^^^^^^^^ > In the A Vs B simple measurement introspection situation one is not > taking into consideration an action but just being a silly memory > fragmentation voyeur. > > > e.g., > > pageblock, would be the best candidate for reclaim and/or compaction. > > That would require a ranking model, e.g., a cost function and weights > > for reclaim and compaction operations, and calculations of the cost to > > produce a free area of a requested order for each pageblock, i.e., a > > 2-dimensional measure > > costs_to_produce_free_area[NR_non_zero_orders][NR_pageblocks]. > > This all makse sense! Thank you!
