On 11/16/21 12:21 PM, Vito Caputo wrote: > On Tue, Nov 16, 2021 at 11:55:41AM -0700, Jens Axboe wrote: >> On 11/16/21 11:36 AM, Matthew Wilcox wrote: >>> On Mon, Nov 15, 2021 at 08:35:30PM -0800, Andrew Morton wrote: >>>> I'd also be interested in seeing feedback from the MM developers. >>> [...] >>>> Subject: Increase default MLOCK_LIMIT to 8 MiB >>> >>> On the one hand, processes can already allocate at least this much >>> memory that is non-swappable, just by doing things like opening a lot of >>> files (allocating struct file & fdtable), using a lot of address space >>> (allocating page tables), so I don't have a problem with it per se. >>> >>> On the other hand, 64kB is available on anything larger than an IBM XT. >>> Linux will still boot on machines with 4MB of RAM (eg routers). For >>> someone with a machine with only, say, 32MB of memory, this allows a >>> process to make a quarter of the memory unswappable, and maybe that's >>> not a good idea. So perhaps this should scale over a certain range? >>> >>> Is 8MB a generally useful amount of memory for an iouring user anyway? >>> If you're just playing with it, sure, but if you have, oh i don't know, >>> a database, don't you want to pin the entire cache and allow IO to the >>> whole thing? >> >> 8MB is plenty for most casual use cases, which is exactly the ones that >> we want to "just work" without requiring weird system level >> modifications to increase the memlock limit. >> > > Considering a single fullscreen 32bpp 4K-resolution framebuffer is > ~32MiB, I'm not convinced this is really correct in nearly 2022. You don't need to register any buffers, and I don't expect any basic uses cases to do so. Which means that the 8MB just need to cover the ring itself, and you can fit a _lot_ of rings into 8MB. The memlock limit only applies to buffers if you register them, not for any "normal" use cases where you just pass buffers for read/write or O_DIRECT read/write. > If we're going to bump the default at the kernel, I'm with Matthew on > making it autoscale within a sane range, depending on available > memory. I just don't want to turn this into a bikeshedding conversation. I'm fine with making it autoscale obviously, but who's going to do the work? > As an upper bound I'd probably look at the highest anticipated > consumer resolutions, and handle a couple fullscreen 32bpp instances > being pinned. Not sure I see the relevance here. -- Jens Axboe
