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Davidlohr Bueso 15df03c879 sysvipc: make get_maxid O(1) again 
For a custom microbenchmark on a 3.30GHz Xeon SandyBridge, which calls
IPC_STAT over and over, it was calculated that, on avg the cost of
ipc_get_maxid() for increasing amounts of keys was:

 10 keys: ~900 cycles
 100 keys: ~15000 cycles
 1000 keys: ~150000 cycles
 10000 keys: ~2100000 cycles

This is unsurprising as maxid is currently O(n).

By having the max_id available in O(1) we save all those cycles for each
semctl(_STAT) command, the idr_find can be expensive -- which some real
(customer) workloads actually poll on.

Note that this used to be the case, until commit 7ca7e564e0 ("ipc:
store ipcs into IDRs").  The cost is the extra idr_find when doing
RMIDs, but we simply go backwards, and should not take too many
iterations to find the new value.

[akpm@linux-foundation.org: coding-style fixes]
Link: http://lkml.kernel.org/r/20170831172049.14576-5-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-11-17 16:10:04 -08:00
..
compat.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
ipc_sysctl.c ipc/msg: increase MSGMNI, remove scaling 2014-12-13 12:42:52 -08:00
Makefile License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
mq_sysctl.c ipc: convert use of typedef ctl_table to struct ctl_table 2014-06-06 16:08:16 -07:00
mqueue.c ipc: mqueue: Replace timespec with timespec64 2017-09-03 20:21:24 -04:00
msg.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
msgutil.c ipc: convert ipc_namespace.count from atomic_t to refcount_t 2017-09-08 18:26:51 -07:00
namespace.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
sem.c sysvipc: duplicate lock comments wrt ipc_addid() 2017-11-17 16:10:04 -08:00
shm.c sysvipc: duplicate lock comments wrt ipc_addid() 2017-11-17 16:10:04 -08:00
syscall.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
util.c sysvipc: make get_maxid O(1) again 2017-11-17 16:10:04 -08:00
util.h sysvipc: make get_maxid O(1) again 2017-11-17 16:10:04 -08:00