Re: [PATCH 17/24] sched/fair: Implement delayed dequeue

Phil Auld
2024-11-05T15:55:57Z

Hi Christian,

On Tue, Nov 05, 2024 at 09:53:49AM +0000 Christian Loehle wrote:
> On 11/4/24 12:50, Phil Auld wrote:
> >
> > Hi Dietmar,
> >
> > On Mon, Nov 04, 2024 at 10:28:37AM +0100 Dietmar Eggemann wrote:
> >> Hi Phil,
> >>
> >> On 01/11/2024 13:47, Phil Auld wrote:
> >>>
> >>> Hi Peterm
> >>>
> >>> On Sat, Jul 27, 2024 at 12:27:49PM +0200 Peter Zijlstra wrote:
> >>>> Extend / fix 86bfbb7ce4f6 ("sched/fair: Add lag based placement") by
> >>>> noting that lag is fundamentally a temporal measure. It should not be
> >>>> carried around indefinitely.
> >>>>
> >>>> OTOH it should also not be instantly discarded, doing so will allow a
> >>>> task to game the system by purposefully (micro) sleeping at the end of
> >>>> its time quantum.
> >>>>
> >>>> Since lag is intimately tied to the virtual time base, a wall-time
> >>>> based decay is also insufficient, notably competition is required for
> >>>> any of this to make sense.
> >>>>
> >>>> Instead, delay the dequeue and keep the 'tasks' on the runqueue,
> >>>> competing until they are eligible.
> >>>>
> >>>> Strictly speaking, we only care about keeping them until the 0-lag
> >>>> point, but that is a difficult proposition, instead carry them around
> >>>> until they get picked again, and dequeue them at that point.
> >>>
> >>> This one is causing a 10-20% performance hit on our filesystem tests.
> >>>
> >>> On 6.12-rc5 (so with the latest follow ons) we get:
> >>>
> >>> with DELAY_DEQUEUE the bandwidth is 510 MB/s
> >>> with NO_DELAY_DEQUEUE the bandwidth is 590 MB/s
> >>>
> >>> The test is fio, something like this:
> >>>
> >>> taskset -c 1,2,3,4,5,6,7,8 fio --rw randwrite --bs 4k --runtime 1m --fsync 0 --iodepth 32 --direct 1 --ioengine libaio --numjobs 8 --size 30g --nrfiles 1 --loops 1 --name default --randrepeat 1 --time_based --group_reporting --directory /testfs
> >>
> >> I'm not seeing this on my i7-13700K running tip sched/core (1a6151017ee5
> >> - sched: psi: pass enqueue/dequeue flags to psi callbacks directly
> >> (2024-10-26 Johannes Weiner)) (6.12.0-rc4 - based)
> >>
> >> Using 'taskset 0xaaaaa' avoiding SMT and running only on P-cores.
> >>
> >> vanilla features: 990MB/s (mean out of 5 runs, σ: 9.38)
> >> NO_DELAY_DEQUEUE: 992MB/s (mean out of 5 runs, σ: 10.61)
> >>
> >> # sudo lshw -class disk -class storage
> >> *-nvme
> >> description: NVMe device
> >> product: GIGABYTE GP-ASM2NE6500GTTD
> >> vendor: Phison Electronics Corporation
> >> physical id: 0
> >> bus info: pci@0000:01:00.0
> >> logical name: /dev/nvme0
> >> version: EGFM13.2
> >> ...
> >> capabilities: nvme pciexpress msix msi pm nvm_express bus_master cap_list
> >> configuration: driver=nvme latency=0 nqn=nqn.2014.08.org.nvmexpress:19871987SN215108954872 GIGABYTE GP-ASM2NE6500GTTD state=live
> >> resources: irq:16 memory:70800000-70803fff
> >>
> >> # mount | grep ^/dev/nvme0
> >> /dev/nvme0n1p2 on / type ext4 (rw,relatime,errors=remount-ro)
> >>
> >> Which disk device you're using?
> >
> > Most of the reports are on various NVME drives (samsung mostly I think).
> >
> >
> > One thing I should add is that it's all on LVM:
> >
> >
> > vgcreate vg /dev/nvme0n1 -y
> > lvcreate -n thinMeta -L 3GB vg -y
> > lvcreate -n thinPool -l 99%FREE vg -y
> > lvconvert --thinpool /dev/mapper/vg-thinPool --poolmetadata /dev/mapper/vg-thinMeta -Zn -y
> > lvcreate -n testLV -V 1300G --thinpool thinPool vg
> > wipefs -a /dev/mapper/vg-testLV
> > mkfs.ext4 /dev/mapper/vg-testLV -E lazy_itable_init=0,lazy_journal_init=0 -F
> > mount /dev/mapper/vg-testLV /testfs
> >
> >
> > With VDO or thinpool (as above) it shows on both ext4 and xfs. With fs on
> > drive directly it's a little more variable. Some it shows on xfs, some it show
> > on ext4 and not vice-versa, seems to depend on the drive or hw raid. But when
> > it shows it's 100% reproducible on that setup.
> >
> > It's always the randwrite numbers. The rest look fine.
>
> Hi Phil,
>
> Thanks for the detailed instructions. Unfortunately even with your LVM setup on
> the platforms I've tried I don't see a regression so far, all the numbers are
> about equal for DELAY_DEQUEUE and NO_DELAY_DEQUEUE.
>

Yeah, that's odd.

Fwiw:

Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Address sizes: 48 bits physical, 48 bits virtual
Byte Order: Little Endian
CPU(s): 32
On-line CPU(s) list: 0-31
Vendor ID: AuthenticAMD
BIOS Vendor ID: Advanced Micro Devices, Inc.
Model name: AMD EPYC 7313P 16-Core Processor
BIOS Model name: AMD EPYC 7313P 16-Core Processor Unknown CPU @ 3.0GHz
BIOS CPU family: 107
CPU family: 25
...

16 SMT2 cores (siblings are 16-31)

#lsblk -N
NAME TYPE MODEL SERIAL REV TRAN RQ-SIZE MQ
nvme3n1 disk SAMSUNG MZQL21T9HCJR-00A07 S64GNJ0T605178 GDC5602Q nvme 1023 32
nvme2n1 disk SAMSUNG MZQL21T9HCJR-00A07 S64GNJ0T605128 GDC5602Q nvme 1023 32
nvme0n1 disk SAMSUNG MZQL21T9HCJR-00A07 S64GNJ0T605125 GDC5602Q nvme 1023 32
nvme1n1 disk SAMSUNG MZQL21T9HCJR-00A07 S64GNJ0T605127 GDC5602Q nvme 1023 32

Where nvme0n1 is the one I'm actually using.

I'm on 6.12.0-0.rc5.44.eln143.x86_64 which is v6.12-rc5 with RHEL .config. This
should have little to no franken-kernel bits but now that I have the machine
I'll build from upstream (with the RHEL .config still) to make sure.

We did see it on all the RCs so far.

> Anyway I have some follow-ups, first let me trim the fio command for readability:
> fio --rw randwrite --bs 4k --runtime 1m --fsync 0 --iodepth 32 --direct 1 --ioengine libaio --numjobs 8 --size 30g --nrfiles 1 --loops 1 --name default --randrepeat 1 --time_based --group_reporting --directory /testfs
>
> dropping defaults nr_files, loops, fsync, randrepeat
> fio --rw randwrite --bs 4k --runtime 1m --iodepth 32 --direct 1 --ioengine libaio --numjobs 8 --size 30g --name default --time_based --group_reporting --directory /testfs
>
> Adding the CPU affinities directly:
> fio --cpus_allowed 1-8 --rw randwrite --bs 4k --runtime 1m --iodepth 32 --direct 1 --ioengine libaio --numjobs 8 --size 30g --name default --time_based --group_reporting --directory /testfs
>

Fair enough. It should work the same with taskset I suppose except the below bit. I've
been given this from our perforance team. They have a framework that produces nice html
pages with red and green results and graphs and whatnot. Right now it's in the form of
a script that pulls the KB/s number of out the json output which is nice and keeps me
from going crosseyed looking at the full fio run output.

> Now I was wondering about the following:
> Is it actually the kworker (not another fio) being preempted? (I'm pretty sure it is)
> To test: --cpus_allowed_policy split (each fio process gets it's own CPU).

with --cpus-allowed and --cpus_allowed_policy split the results with DELAY_DEQUEUE are
better (540MB/s) but with NO_DELAY_DEQUEUE they are also better (640 MB/s). It was
510MB/s and 590MB/s before.

>
> You wrote:
> >I was thinking maybe the preemption was preventing some batching of IO completions or
> >initiations. But that was wrong it seems.
>
> So while it doesn't reproduce for me, the only thing being preempted regularly is
> the kworker (running iomap_dio_complete_work). I don't quite follow the "that was
> wrong it seems" part then. Could you elaborate?
>

I was thinking that the fio batch test along with the disabling WAKEUP_PREEMPTION
was telling me that it wasn't the over preemption issue, but that also I could be
wrong about...

> Could you also post the other benchmark numbers? Does any of them score higher in IOPS?
> Is --rw write the same issue if you set --bs 4k (assuming you set a larger bs for seqwrite).
>

I don't have numbers for all of the other flavors but I ran --rw write --bs 4k:

DELAY_DEQUEUE ~590MB/s
NO_DELAY_DEQUEUE ~840MB/s

Those results are not good for DELAY_DEQUEUE either.

> Can you set the kworkers handling completions to SCHED_BATCH too? Just to confirm.

I think I did the wrong kworkes the first time. So I'll try again to figure out which
kworkers to twiddle (or I'll just do all 227 of them...).

Thanks,
Phil

>
> Regards,
> Christian
>

--