Re: [PATCH] Increase default MLOCK_LIMIT to 8 MiB

Jens Axboe
2021-11-16T19:41:04Z

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