From: Rasmus Villemoes [mailto:linux@xxxxxxxxxxxxxxxxxx] > On Fri, Dec 16 2016, Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> 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. ��.n��������+%������w��{.n�����{����n�r������&��z�ޗ�zf���h���~����������_��+v���)ߣ�
