There is a feature inside of both schedutil and intel_pstate called
iowait boosting which tries to prevent selecting a low frequency
during IO workloads when it impacts throughput.
The feature is implemented by checking for task wakeups that have
the in_iowait flag set and boost the CPU of the rq accordingly
(implemented through cpufreq_update_util(rq, SCHED_CPUFREQ_IOWAIT)).
The necessity of the feature is argued with the potentially low
utilization of a task being frequently in_iowait (i.e. most of the
time not enqueued on any rq and cannot build up utilization).
The RFC focuses on the schedutil implementation.
intel_pstate frequency selection isn't touched for now, suggestions are
very welcome.
Current schedutil iowait boosting has several issues:
1. Boosting happens even in scenarios where it doesn't improve
throughput. [1]
2. The boost is not accounted for in EAS: a) feec() will only consider
the actual utilization for task placement, but another CPU might be
more energy-efficient at that capacity than the boosted one.)
b) When placing a non-IO task while a CPU is boosted compute_energy()
will not consider the (potentially 'free') boosted capacity, but the
one it would have without the boost (since the boost is only applied
in sugov).
3. Actual IO heavy workloads are hardly distinguished from infrequent
in_iowait wakeups.
4. The boost isn't associated with a task, it therefore isn't considered
for task placement, potentially missing out on higher capacity CPUs on
heterogeneous CPU topologies.
5. The boost isn't associated with a task, it therefore lingers on the
rq even after the responsible task has migrated / stopped.
6. The boost isn't associated with a task, it therefore needs to ramp
up again when migrated.
7. Since schedutil doesn't know which task is getting woken up,
multiple unrelated in_iowait tasks might lead to boosting.
We attempt to mitigate all of the above by reworking the way the
iowait boosting (io boosting from here on) works in two major ways:
- Carry the boost in task_struct, so it is a per-task attribute and
behaves similar to utilization of the task in some ways.
- Employ a counting-based tracking strategy that only boosts as long
as it sees benefits and returns to no boosting dynamically.
Note that some the issues (1, 3) can be solved by using a
counting-based strategy on a per-rq basis, i.e. in sugov entirely.
Experiments with Android in particular showed that such a strategy
(which necessarily needs longer intervals to be reasonably stable)
is too prone to migrations to be useful generally.
We therefore consider the additional complexity of such a per-task
based approach like proposed to be worth it.
We require a minimum of 1000 iowait wakeups per second to start
boosting.
This isn't too far off from what sugov currently does, since it resets
the boost if it hasn't seen an iowait wakeup for TICK_NSEC.
For CONFIG_HZ=1000 we are on par, for anything below we are stricter.
We justify this by the small possible improvement by boosting in the
first place with 'rare' few iowait wakeups.
When IO even leads to a task being in iowait isn't as straightforward
to explain.
Of course if the issued IO can be served by the page cache (e.g. on
reads because the pages are contained, on writes because they can be
marked dirty and the writeback takes care of it later) the actual
issuing task is usually not in iowait.
We consider this the good case, since whenever the scheduler and a
potential userspace / kernel switch is in the critical path for IO
there is possibly overhead impacting throughput.
We therefore focus on random read from here on, because (on synchronous
IO [3]) this will lead to the task being set in iowait for every IO.
This is where iowait boosting shows its biggest throughput improvement.
>From here on IOPS (IO operations per second) and iowait wakeups may
therefore be used interchangeably.
Performance:
Throughput for random read tries to be on par with the sugov
implementation of iowait boosting for reasonably long-lived workloads.
See the following table for some results, values are in IOPS, the
tests are ran for 30s with pauses in-between, results are sorted.
nvme on rk3399
[3588, 3590, 3597, 3632, 3745] sugov mainline
[3581, 3751, 3770, 3771, 3885] per-task tracking
[2592, 2639, 2701, 2717, 2784] sugov no iowait boost
[3218, 3451, 3598, 3848, 3921] performance governor
emmc with cqe on rk3399
[4146, 4155, 4159, 4161, 4193] sugov mainline
[2848, 3217, 4375, 4380, 4454] per-task tracking
[2510, 2665, 3093, 3101, 3105] sugov no iowait boost
[4690, 4803, 4860, 4976, 5069] performance governor
sd card on rk3399
[1777, 1780, 1806, 1827, 1850] sugov mainline
[1470, 1476, 1507, 1534, 1586] per-task tracking
[1356, 1372, 1373, 1377, 1416] sugov no iowait boost
[1861, 1890, 1901, 1905, 1908] performance governor
Pixel 6 ufs Android 14 (7 runs for because device showed some variance)
[6605, 6622, 6633, 6652, 6690, 6697, 6754] sugov mainline
[7141, 7173, 7198, 7220, 7280, 7427, 7452] per-task tracking
[2390, 2392, 2406, 2437, 2464, 2487, 2813] sugov no iowait boost
[7812, 7837, 7837, 7851, 7900, 7959, 7980] performance governor
Apple M1 apple-nvme
[27421, 28331, 28515, 28699, 29529] sugov mainline
[27274, 27344, 27345, 27384, 27930] per-task tracking
[14480, 14512, 14625, 14872, 14967] sugov no iowait boost
[31595, 32085, 32386, 32465, 32643] performance governor
Showcasing some different IO scenarios, again all random read,
median out of 5 runs, all on rk3399 with NVMe.
e.g. io_uring6x4 means 6 threads with 4 iodepth each, results can be
obtained using:
fio --minimal --time_based --name=test --filename=/dev/nvme0n1 --runtime=30 --rw=randread --bs=4k --ioengine=io_uring --iodepth=4 --numjobs=6 --group_reporting | cut -d \; -f 8
+---------------+----------------+-------------------+----------------+-------------+-----------+
| | Sugov mainline | Per-task tracking | Sugov no boost | Performance | Powersave |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| psyncx1 | 4073 | 3793 | 2979 | 4190 | 2788 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| psyncx4 | 13921 | 13503 | 10635 | 13931 | 10225 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| psyncx6 | 18473 | 17866 | 15902 | 19080 | 15789 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| psyncx8 | 22498 | 21242 | 19867 | 22650 | 18837 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| psyncx10 | 24801 | 23552 | 23658 | 25096 | 21474 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| psyncx12 | 26743 | 25377 | 26372 | 26663 | 23613 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| libaio1x1 | 4054 | 3542 | 2776 | 4055 | 2780 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| libaio1x128 | 3959 | 3516 | 2758 | 3590 | 2560 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| libaio4x128 | 13451 | 12517 | 10313 | 13403 | 9994 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| libaio6x1 | 18394 | 17432 | 15340 | 18954 | 15251 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| libaio6x4 | 18329 | 17100 | 15238 | 18623 | 15270 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| libaio6x128 | 18066 | 16964 | 15139 | 18577 | 15192 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring1x1 | 4043 | 3548 | 2810 | 4039 | 2689 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring4x64 | 35790 | 32814 | 35983 | 34934 | 33254 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring1x128 | 32651 | 30427 | 32429 | 33232 | 9973 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring2x128 | 34928 | 32595 | 34922 | 33726 | 18790 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring4x128 | 34414 | 32173 | 34932 | 33332 | 33005 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring6x4 | 31578 | 29260 | 31714 | 31399 | 31784 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
| io_uring6x128 | 34480 | 32634 | 34973 | 33390 | 36452 |
+---------------+----------------+-------------------+----------------+-------------+-----------+
Based on the above we can basically categorize these into the following
three:
a) boost is useful
b) boost irrelevant (util dominates)
c) boost is energy-inefficient (boost dominates)
The aim of the patch 1/2 is to boost as much as necessary for a) while
boosting little for c) (thus saving energy).
Energy-savings:
Regarding sugov iowait boosting problem 1 mentioned earlier,
some improvement can be seen:
Tested on rk3399 (LLLL)(bb) with an NVMe, 30s runtime
CPU0 perf domain spans 0-3 with 400MHz to 1400MHz
CPU4 perf domain spans 4-5 with 400MHz to 1800MHz
io_uring6x128:
Sugov iowait boost:
Average frequency for CPU0 : 1.180 GHz
Average frequency for CPU4 : 1.504 GHz
Per-task tracking:
Average frequency for CPU0 : 1.070 GHz
Average frequency for CPU4 : 1.211 GHz
io_uring12x128:
Sugov iowait boost:
Average frequency for CPU0 : 1.324 GHz
Average frequency for CPU4 : 1.444 GHz
Per-task tracking:
Average frequency for CPU0 : 1.260 GHz
Average frequency for CPU4 : 1.062 GHz
(In both cases actually 400MHz on both perf domains is optimal, more
fine-tuning could get us closer [2])
[1]
There are many scenarios when it doesn't, so let's start with
explaining when it does:
Boosting improves throughput if there is frequent IO to a device from
one or few origins, such that the device is likely idle when the task
is enqueued on the rq and reducing this time cuts down on the storage
device idle time.
This might not be true (and boosting doesn't help) if:
- The storage device uses the idle time to actually commit the IO to
persistent storage or do other management activity (this can be
observed with e.g. writes to flash-based storage, which will usually
write to cache and flush the cache when idle or necessary).
- The device is under thermal pressure and needs idle time to cool off
(not uncommon for e.g. nvme devices).
Furthermore the assumption (the device being idle while task is
enqueued) is false altogether if:
- Other tasks use the same storage device.
- The task uses asynchronous IO with iodepth > 1 like io_uring, the
in_iowait is then just to fill the queue on the host again.
- The task just sets in_iowait to signal it is waiting on io to not
appear as system idle, it might not send any io at all (cf with
the various occurrences of in_iowait, io_mutex_lock and io_schedule*).
[3]
Unfortunately even for asynchronous IO iowait may be set, in the case
of io_uring this is specifically for the iowait boost to trigger, see
commit ("8a796565cec3 io_uring: Use io_schedule* in cqring wait")
which is why the energy-savings are so significant here, as io_uring
load on the CPU is minimal.
Problems encountered:
- Higher cap is not always beneficial, we might place the task away
from the CPU where the interrupt handler is running, making it run
on an unboosted CPU which may have a bigger impact than the difference
between the CPU's capacity the task moved to. (Of course the boost will
then be reverted again, but a ping-pong every interval is possible).
- [2] tracking and scaling can be improved (io_uring12x128 still shows
boosting): Unfortunately tracking purely per-task shows some limits.
One task might show more iowaits per second when boosted, but overall
throughput doesn't increase => there is still some boost.
The task throughput improvement is somewhat limited though,
so by fine-tuning the thresholds there could be mitigations.
Christian Loehle (2):
sched/fair: Introduce per-task io util boost
cpufreq/schedutil: Remove iowait boost
include/linux/sched.h | 15 +++
kernel/sched/cpufreq_schedutil.c | 150 ++--------------------------
kernel/sched/fair.c | 165 +++++++++++++++++++++++++++++--
kernel/sched/sched.h | 4 +-
4 files changed, 182 insertions(+), 152 deletions(-)
--
2.34.1
