[RFC 00/10] implement alternative and much simpler id allocator

[Matthew Wilcox]

From: Rasmus Villemoes [mailto:linux at rasmusvillemoes.dk]
> On Fri, Dec 16 2016, Matthew Wilcox <mawilcox at microsoft.com> wrote:
> > Thanks for your work on this; you've really put some effort into
> > proving your work has value.  My motivation was purely aesthetic, but
> > you've got some genuine savings here (admittedly it's about a quarter
> > of a cent's worth of memory with DRAM selling for $10/GB).
> > Nevertheless, that adds up over a billion devices, and there are still
> > people trying to fit Linux into 4MB embedded devices.
> >
> 
> Yeah, my main motivation was embedded devices which don't have the
> luxury of measuring their RAM in GB. E.g., it's crazy that the
> watchdog_ida effectively use more memory than the .text of the watchdog
> subsystem, and similarly for the kthread workers, etc., etc.. I didn't
> mean for my patches to go in as is, more to provoke some discussion. I
> wasn't aware of your reimplementation, but it seems that may make the
> problem go away.

It certainly shrinks the problem down to a size where it may not be worth introducing another implementation.

> > On a 64-bit machine, your tIDA root is 24 bytes; my new IDA root is 16
> > bytes.  If you allocate only one entry, you'll allocate 8 bytes.
> > Thanks to the slab allocator, that gets rounded up to 32 bytes.  I
> > allocate the full 128 byte leaf, but I store the pointer to it in the
> > root (unlike the IDR, the radix tree doesn't need to allocate a layer
> > for a single entry).  So tIDA wins on memory consumption between 1 and
> > 511 IDs, and newIDA is slightly ahead between 512 and 1023 IDs.
> 
> This sounds good. I think there may still be a lot of users that never
> allocate more than a handful of IDAs, making a 128 byte allocation still
> somewhat excessive. One thing I considered was (exactly as it's done for
> file descriptor tables) to embed a single word in the struct ida and
> use that initially; I haven't looked closely at newIDA, so I don't know
> how easy that would be or if its worth the complexity.

Heh, I was thinking about that too.  The radix tree supports "exceptional entries" which have the bottom bit set.  On a 64-bit machine, we could use 62 of the bits in the radix tree root to store the ID bitmap.  I'm a little wary of the potential complexity, but we should try it out.

Did you come up with any fun tests that could be added to the test-suite?  It feels a little slender right now.