During some io test, I found that waitqueues can be extremly unbalanced,
especially when tags are little.
For example:
test cmd: nr_requests is set to 64, and queue_depth is set to 32
[global]
filename=/dev/sdh
ioengine=libaio
direct=1
allow_mounted_write=0
group_reporting
[test]
rw=randwrite
bs=4k
numjobs=512
iodepth=2
With patch 1 applied, I observe the following status:
ws_active=484
ws={
{.wait_cnt=8, .waiters_cnt=117},
{.wait_cnt=8, .waiters_cnt=59},
{.wait_cnt=8, .waiters_cnt=76},
{.wait_cnt=8, .waiters_cnt=0},
{.wait_cnt=5, .waiters_cnt=24},
{.wait_cnt=8, .waiters_cnt=12},
{.wait_cnt=8, .waiters_cnt=21},
{.wait_cnt=8, .waiters_cnt=175},
}
'waiters_cnt' means how many threads are waitng for tags in the 'ws',
and such extremely unbalanced status is very frequent. After reading the
sbitmap code, I found there are two situations that might cause the
problem:
1) blk_mq_get_tag() can call 'bt_wait_ptr()' while the threads might get
tag successfully before going to wait. - patch 2
2) After a 'ws' is woken up, following blk_mq_put_tag() might wake up
the same 'ws' again instead of the next one. - patch 3
I'm not sure if the unbalanced status is really a *problem* and need to
be fixed, this patchset is just to improve fairness and not a thorough
fix. Any comments and suggestions are welcome.
Yu Kuai (3):
sbitmap: record the number of waiters for each waitqueue
blk-mq: call 'bt_wait_ptr()' later in blk_mq_get_tag()
sbitmap: improve the fairness of waitqueues' wake up
block/blk-mq-tag.c | 6 ++---
include/linux/sbitmap.h | 5 ++++
lib/sbitmap.c | 57 ++++++++++++++++++++++-------------------
3 files changed, 39 insertions(+), 29 deletions(-)
--
2.31.1
