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

Rasmus Villemoes linux at rasmusvillemoes.dk
Fri Dec 16 20:32:11 UTC 2016

On Fri, Dec 16 2016, Matthew Wilcox <mawilcox at microsoft.com> wrote:

> From: Andrew Morton [mailto:akpm at linux-foundation.org]
>> On Thu,  8 Dec 2016 02:22:55 +0100 Rasmus Villemoes
>> <linux at rasmusvillemoes.dk> wrote:
>> > TL;DR: these patches save 250 KB of memory, with more low-hanging
>> > fruit ready to pick.
>> >
>> > While browsing through the lib/idr.c code, I noticed that the code at
>> > the end of ida_get_new_above() probably doesn't work as intended: Most
>> > users of ida use it via ida_simple_get(), and that starts by
>> > unconditionally calling ida_pre_get(), ensuring that ida->idr has
>> > 8==MAX_IDR_FREE idr_layers in its free list id_free. In the common
>> > case, none (or at most one) of these get used during
>> > ida_get_new_above(), and we only free one, leaving at least 6 (usually
>> > 7) idr_layers in the free list.
>> I expect we'll be merging patches 1-32 of that series into 4.10-rc1 and
>> the above patch (#33) into 4.11-rc1.
> Hi Rasmus,
> 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.

> I think my reimplementation of the IDA on top of the radix tree is
> close enough to your tIDA in memory consumption that it doesn't
> warrant a new data structure.
> 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.


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