I’d been impressed by L4. It’s certainly been deployed pretty broadly.
And it has recursive pagers but … not sure how that’s used in practice. And there are a bunch of variants. Sent from my iPhone > On Jan 8, 2026, at 9:23 PM, [email protected] wrote: > > I vaguely remember someone being quoted as saying > > Microkernels don't have to be small. They just have to > not do much. > > :-) > > ron minnich <[email protected]> wrote: > >> I would not tar the idea of external pagers with the Mach tarbrush. Mach >> was pretty much inefficient at everything, including external pagers. >> External pagers can work well, when implemented well. >> >>> On Thu, Jan 8, 2026 at 8:41 PM Paul Lalonde <[email protected]> >>> wrote: >>> >>> Did the same on GPUs/Xeon Phi, including in the texture units. Very >>> useful mechanism for abstracting compute with random access characteristics. >>> >>> Paul >>> >>>> On Wed, Jan 7, 2026, 1:35 p.m. ron minnich <[email protected]> wrote: >>>> what we had planned for harvey was a good deal simpler: designate a part >>>> of the address space as a "bounce fault to user" space area. >>>> >>>> When a page fault in that area occurred, info about the fault was sent to >>>> an fd (if it was opened) or a note handler. >>>> >>>> user could could handle the fault or punt, as it saw fit. The fixup was >>>> that user mode had to get the data to satisfy the fault, then tell the >>>> kernel what to do. >>>> >>>> This is much like the 35-years-ago work we did on AIX, called >>>> external pagers at the time; or the more recent umap work, >>>> https://computing.llnl.gov/projects/umap, used fairly widely in HPC. >>>> >>>> If you go this route, it's a bit less complex than what you are proposing. >>>> >>>> On Wed, Jan 7, 2026 at 1:09 PM Bakul Shah via 9fans <[email protected]> >>>> wrote: >>>> >>>>> >>>>> >>>>>> On Jan 7, 2026, at 8:41 AM, [email protected] wrote: >>>>>> >>>>>> Quoth Bakul Shah via 9fans <[email protected]>: >>>>>>> I have this idea that will horrify most of you! >>>>>>> >>>>>>> 1. Create an mmap device driver. You ask it to a new file handle >>>>> which you use to communicate about memory mapping. >>>>>>> 2. If you want to mmap some file, you open it and write its file >>>>> descriptor along with other parameters (file offset, base addr, size, >>>>> mode, >>>>> flags) to your mmap file handle. >>>>>>> 3. The mmap driver sets up necessary page table entries but doesn't >>>>> actually fetch any data before returning from the write. >>>>>>> 4. It can asynchronously kick off io requests on your behalf and >>>>> fixup page table entries as needed. >>>>>>> 5. Page faults in the mmapped area are serviced by making appropriate >>>>> read/write calls. >>>>>>> 6. Flags can be used to indicate read-ahead or write-behind for >>>>> typical serial access. >>>>>>> 7. Similarly msync, munmap etc. can be implemented. >>>>>>> >>>>>>> In a sneaky way this avoids the need for adding any mmap specific >>>>> syscalls! But the underlying work would be mostly similar in either case. >>>>>>> >>>>>>> The main benefits of mmap are reduced initial latency , "pay as you >>>>> go" cost structure and ease of use. It is certainly more expensive than >>>>> reading/writing the same amount of data directly from a program. >>>>>>> >>>>>>> No idea how horrible a hack is needed to implement such a thing or >>>>> even if it is possible at all but I had to share this ;-) >>>>>> >>>>>> To what end? The problems with mmap have little to do with adding a >>>>> syscall; >>>>>> they're about how you do things like communicating I/O errors. >>>>> Especially >>>>>> when flushing the cache. >>>>>> >>>>>> Imagine the following setup -- I've imported 9p.io: >>>>>> >>>>>> 9fs 9pio >>>>>> >>>>>> and then I map a file from it: >>>>>> >>>>>> mapped = mmap("/n/9pio/plan9/lib/words", OWRITE); >>>>>> >>>>>> Now, I want to write something into the file: >>>>>> >>>>>> *mapped = 1234; >>>>>> >>>>>> The cached version of the page is dirty, so the OS will >>>>>> eventually need to flush it back with a 9p Twrite; Let's >>>>>> assume that before this happens, the network goes down. >>>>>> >>>>>> How do you communicate the error with userspace? >>>>> >>>>> This was just a brainwave but... >>>>> >>>>> You have a (control) connection with the mmap device to >>>>> set up mmap so might as well use it to convey errors! >>>>> This device would be strictly local to where a program >>>>> runs. >>>>> >>>>> I'd even consider allowing a separate process to mmap, >>>>> by making an address space a first class object. That'd >>>>> move more stuff out of the kernel and allow for more >>>>> interesting/esoteric uses. >>>> *9fans <https://9fans.topicbox.com/latest>* / 9fans / see discussions >>> <https://9fans.topicbox.com/groups/9fans> + participants >>> <https://9fans.topicbox.com/groups/9fans/members> + delivery options >>> <https://9fans.topicbox.com/groups/9fans/subscription> Permalink >>> <https://9fans.topicbox.com/groups/9fans/Te8d7c6e48b5c075b-Mf3cfeeb18fd00292d3f9063f> >>> ------------------------------------------ 9fans: 9fans Permalink: https://9fans.topicbox.com/groups/9fans/Te8d7c6e48b5c075b-M4dfecc367000953ec3cd500e Delivery options: https://9fans.topicbox.com/groups/9fans/subscription
