On Fri, 31 Jan 2025 17:51:45 +0000 Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote: > On Fri, Jan 31, 2025 at 12:47:24PM -0500, Liam R. Howlett wrote: > > * Davidlohr Bueso <dave@xxxxxxxxxxxx> [250131 12:31]: > > > On Fri, 31 Jan 2025, Lorenzo Stoakes wrote: > > > > > > > On Thu, Jan 16, 2025 at 05:30:58PM -0800, SeongJae Park wrote: > > > > > Optimize redundant mmap lock operations from process_madvise() by > > > > > directly doing the mmap locking first, and then the remaining works for > > > > > all ranges in the loop. > > > > > > > > > > Signed-off-by: SeongJae Park <sj@xxxxxxxxxx> > > > > > > > > I wonder if this might increase lock contention because now all of the > > > > vector operations will hold the relevant mm lock without releasing after > > > > each operation? > > > > > > That was exactly my concern. While afaict the numbers presented in v1 > > > are quite nice, this is ultimately a micro-benchmark, where no other > > > unrelated threads are impacted by these new hold times. > > > > Indeed, I was also concerned about this scenario. > > > > But this method does have the added advantage of keeping the vma space > > in the same state as it was expected during the initial call - although > > the race does still exist on looking vs acting on the data. This would > > just remove the intermediate changes. > > > > > > > > > Probably it's ok given limited size of iov, I think so. Also, users could adjust the batching size for their workloads. > > > > but maybe in future we'd want > > > > to set a limit on the ranges before we drop/reacquire lock? > > > > > > imo, this should best be done in the same patch/series. Maybe extend > > > the benchmark to use IOV_MAX and find a sweet spot? > > > > Are you worried this is over-engineering for a problem that may never be > > an issue, or is there a particular usecase you have in mind? > > > > It is probably worth investigating, and maybe a potential usecase would > > help with the targeted sweet spot? I think the sweet spot may depend on the workload and the advice type. So selection of the benchmark will be important. Do you have a benchmark in your mind? My humble microbenchmark[1] does only single-thread usage performance evaluation, so may not be appropriate to be used here. I actually do the evaluation with batching size up to the IOV_MAX (1024), but show no clear evidence of the sweet spot. > > > > Keep in mind process_madvise() is not limited by IOV_MAX, which can be rather > high, but rather UIO_FASTIOV, which is limited to 8 entries. process_madvise() indeed have iovec array of UIO_FASTIOV size, namely iovstack. But, if I understood the code correctly, iostack is used only for a fast path that the user requested advicing less than UIO_FASTIOV regions. I actually confirmed I can make the loop itrate 1024 times, using my microbenchmark[1]. My step for the check was running the program with 'eval_pmadv $((4*1024*1024)) $((4*1024)) $((4*1024*1024))' command, and counting the number of the itration of the vector_madvise() main loop using printk(). Please let me know if I'm missing something. > > (Some have been surprised by this limitation...!) > > So I think at this point scaling isn't a huge issue, I raise it because in > future we may want to increase this limit, at which point we should think about > it, which is why I sort of hand-waved it away a bit. I personally think this may still not be a huge issue, especially given the fact that users can test and tune the limit. But I'd like to hear more opinions if available. [1] https://github.com/sjp38/eval_proc_madvise/blob/master/eval_pmadv.c Thanks, SJ > > > Thanks, > > Liam > > >
