On Wed, Mar 15, 2023 at 09:03:57PM +0100, David Hildenbrand wrote: > On 10.03.23 19:28, Stefan Roesch wrote: > > So far KSM can only be enabled by calling madvise for memory regions. To > > be able to use KSM for more workloads, KSM needs to have the ability to be > > enabled / disabled at the process / cgroup level. > > > > Use case 1: > > The madvise call is not available in the programming language. An example for > > this are programs with forked workloads using a garbage collected language without > > pointers. In such a language madvise cannot be made available. > > > > In addition the addresses of objects get moved around as they are garbage > > collected. KSM sharing needs to be enabled "from the outside" for these type of > > workloads. > > > > Use case 2: > > The same interpreter can also be used for workloads where KSM brings no > > benefit or even has overhead. We'd like to be able to enable KSM on a workload > > by workload basis. > > > > Use case 3: > > With the madvise call sharing opportunities are only enabled for the current > > process: it is a workload-local decision. A considerable number of sharing > > opportuniites may exist across multiple workloads or jobs. Only a higler level > > entity like a job scheduler or container can know for certain if its running > > one or more instances of a job. That job scheduler however doesn't have > > the necessary internal worklaod knowledge to make targeted madvise calls. > > > > Security concerns: > > In previous discussions security concerns have been brought up. The problem is > > that an individual workload does not have the knowledge about what else is > > running on a machine. Therefore it has to be very conservative in what memory > > areas can be shared or not. However, if the system is dedicated to running > > multiple jobs within the same security domain, its the job scheduler that has > > the knowledge that sharing can be safely enabled and is even desirable. > > > > Performance: > > Experiments with using UKSM have shown a capacity increase of around 20%. > > Stefan, can you do me a favor and investigate which pages we end up > deduplicating -- especially if it's mostly only the zeropage and if it's > still that significant when disabling THP? > > > I'm currently investigating with some engineers on playing with enabling KSM > on some selected processes (enabling it blindly on all VMAs of that process > via madvise() ). > > One thing we noticed is that such (~50 times) 20MiB processes end up saving > ~2MiB of memory per process. That made me suspicious, because it's the THP > size. > > What I think happens is that we have a 2 MiB area (stack?) and only touch a > single page. We get a whole 2 MiB THP populated. Most of that THP is zeroes. > > KSM somehow ends up splitting that THP and deduplicates all resulting > zeropages. Thus, we "save" 2 MiB. Actually, it's more like we no longer > "waste" 2 MiB. I think the processes with KSM have less (none) THP than the > processes with THP enabled, but I only took a look at a sample of the > process' smaps so far. THP and KSM is indeed an interesting problem. Better TLB hits with THPs, but reduced chance of deduplicating memory - which may or may not result in more IO that outweighs any THP benefits. That said, the service in the experiment referenced above has swap turned on and is under significant memory pressure. Unused splitpages would get swapped out. The difference from KSM was from deduplicating pages that were in active use, not internal THP fragmentation.
