An end to high memory?

Posted Mar 2, 2020 1:05 UTC (Mon) by dave4444 (subscriber, #127523)
Parent article: An end to high memory?

Having worked on many 32bit CPUs with large memory, this is somewhat nistalgic, but it's time is probabaly near the end. And yes, there were many 32bit ARM CPUs with large amounts of memory (such as some NPUs!). Intel also had some SKUs with 32bit cores long after consumer and server chips supported 64bit, notibly some SoCs and Atom CPUs. Deprecating is probably due (especially for some archs).

Lets also not forget about highmem's ugly step child, the bounce buffer for IO devices that DMA to/from high memory. Oh the headaches from that.

An end to high memory?

Posted Mar 5, 2020 18:52 UTC (Thu) by kpfleming (subscriber, #23250) [Link] (4 responses)

Pretty much every Raspberry Pi 4B sold with 2GB or 4GB of RAM is in this situation, because the default (and still the only well-supported) kernel is a 32-bit kernel. There are large numbers of these, and they are being sold and installed today, so can be expected to be in use for quite some time.

An end to high memory?

Posted Mar 6, 2020 9:39 UTC (Fri) by geert (subscriber, #98403) [Link] (3 responses)

But one day the Raspberry Pi 4B will run a 64-bit kernel, solving the problem, right?

An end to high memory?

Posted Mar 6, 2020 10:59 UTC (Fri) by kpfleming (subscriber, #23250) [Link] (2 responses)

It can be done today, if you're willing to give up support for some of the onboard peripherals. Even when there is full hardware support, it's likely that most users won't convert their existing systems.

An end to high memory?

Posted Mar 6, 2020 11:04 UTC (Fri) by geert (subscriber, #98403) [Link] (1 responses)

So they keep on using their current kernel, which includes highmem support.

An end to high memory?

Posted Mar 6, 2020 11:09 UTC (Fri) by kpfleming (subscriber, #23250) [Link]

Sorry, that wasn't quite what I meant to say. I follow the stream of kernel updates provided by the Raspberry Pi foundation, so there are frequent kernel updates. If they can deliver an 'update' from a 32-bit kernel to a 64-bit kernel without requiring significant changes in the userspace tools installed on the machine, then users could be upgraded in place.