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. The comment suggests that the idea is that subsequent allocations would get rid of these one by one, but this doesn't work due to the unconditional filling done by ida_simple_get - and callers who do their own locking presumably call ida_pre_get themselves, with the same effect. To confirm my suspicion, and to see how many idas are actually in use and how much they're utilized, I hacked together a /proc/ida file to provide some stats on the currently active idas. Some examples of lines from that on my laptop: file:line gets puts diff layers id_free_cnt block/blk-core.c:49 27 0 27 1 7 drivers/base/platform.c:35 1 0 1 1 6 drivers/gpu/drm/drm_crtc.c:201 0 0 0 0 0 drivers/gpu/drm/drm_crtc.c:201 0 0 0 0 0 ... drivers/gpu/drm/drm_crtc.c:201 1 0 1 1 6 drivers/gpu/drm/drm_crtc.c:201 2 0 2 1 7 drivers/gpu/drm/drm_crtc.c:5700 5 0 5 1 7 ... drivers/scsi/hosts.c:45 6 0 6 1 7 drivers/scsi/sd.c:123 1 0 1 1 6 fs/devpts/inode.c:386 57 40 17 1 7 fs/kernfs/dir.c:918 10 0 10 1 7 fs/kernfs/dir.c:918 11 0 11 1 7 ... fs/kernfs/dir.c:918 24799 1609 23190 1 7 ... fs/super.c:882 49 5 44 1 7 kernel/workqueue.c:3198 10 8 2 1 7 kernel/workqueue.c:3198 11 9 2 1 7 kernel/workqueue.c:3198 2 0 2 1 7 kernel/workqueue.c:3198 2 0 2 1 7 kernel/workqueue.c:3198 2 0 2 1 7 kernel/workqueue.c:3198 94 91 3 1 7 file:line is the location of the ida_init or DEFINE_IDA, gets and puts are the lifetime number of allocations/frees - their difference is thus the current number of allocated ids. layers and id_free_cnt are the members of ida->idr. As expected, there are 0, 6 or 7 idr_layers cached and unused in each ida, depending on whether there have been 0, 1 or more allocations done. Add the idr_layer which is in use, and we see that e.g. the workqueue instances use more than 6*8*sizeof(struct idr_layer) ~ 100K of memory, or almost 1K per allocated id. Patches 1 and 2 are minor optimization opportunities, while patch 3 is an attempt at making the 'free the extra idr_layers one at a time' actually work. But it's not a very good solution, since it doesn't help the users who never do more than a handful of allocations, nor does it help those who call ida_pre_get/ida_get_new directly. Moreover, even if we somehow had perfect heuristics and got rid of all excess idr_layers and ida_bitmap (another 128 bytes we usually have hanging around), the minimum overhead of sizeof(struct idr_layer)+sizeof(struct ida_bitmap) ~ 2200 bytes is quite a lot for the many ida users who never allocate more than 100 ids. So instead/in addition, I suggest we introduce a much simpler allocator based on a dynamically growing bitmap. Patches 4-10 introduce struct tida and has a few examples of replacing ida with tida. [Yeah, tida is not a good name, and probably _get and _put are also bad.] I've booted a 4.8.12 kernel with these backported, and as expected they save around 250KB of memory. There's more to gain, but this is just an RFC. Rasmus Villemoes (10): lib/idr.c: reused free bitmaps are already clear lib/idr.c: delete useless condition lib/idr.c: only fill ida->idr when needed lib/tida.c: a very simple integer id allocator kernel/workqueue.c: replace id allocator ida with tida block: use tida as small id allocator drivers/base/platform.c: use simpler id allocator lib/tida.c: introduce tida_get_above drm: use simpler id allocator fs/devpts: use tida for id allocation block/blk-core.c | 6 +-- block/blk-sysfs.c | 2 +- block/blk.h | 4 +- drivers/base/platform.c | 10 ++-- drivers/gpu/drm/drm_connector.c | 21 ++++---- drivers/gpu/drm/drm_crtc.c | 4 +- fs/devpts/inode.c | 28 ++++------- include/drm/drm_crtc.h | 3 +- include/linux/tida.h | 32 ++++++++++++ kernel/workqueue.c | 14 +++--- lib/Makefile | 2 +- lib/idr.c | 13 +++-- lib/tida.c | 107 ++++++++++++++++++++++++++++++++++++++++ 13 files changed, 188 insertions(+), 58 deletions(-) create mode 100644 include/linux/tida.h create mode 100644 lib/tida.c -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
