From: Isaac Boukris <iboukris@gmail.com>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: users@dpdk.org
Subject: Re: Accuracy of rte_get_tsc_hz() compared to linux
Date: Thu, 19 Sep 2024 12:37:43 +0300 [thread overview]
Message-ID: <CAC-fF8TRWwn=p+sHO_PBS=A87O28fvmUvR7Mo2f+OR4CTenmCg@mail.gmail.com> (raw)
In-Reply-To: <20240918162739.144ffd9d@hermes.local>
On Thu, Sep 19, 2024 at 2:27 AM Stephen Hemminger
<stephen@networkplumber.org> wrote:
>
> On Thu, 19 Sep 2024 01:04:40 +0300
> Isaac Boukris <iboukris@gmail.com> wrote:
>
> > I've run the helloworld application on an isolated cpu:
> > taskset -c 10 ./dpdk-helloworld --log-level=lib.eal:debug --no-huge
> >
> > The results are:
> > EAL: TSC frequency arch ~2100000 KHz
> > EAL: TSC frequency linux ~2095082 KHz
> > EAL: TSC frequency estimate ~2095346 KHz
> >
> > The arch one is picked, which seems rather wrong, any way to override that?
> > Should we lower the estimation rounding to 1MHz or even 1KHz in the linux one?
> >
> > Thoughts? Thanks!
> >
> > Kernel: 4.18.0-513.9.1.el8_9.x86_64
>
> Note: 4.18 kernel was end of life 12 August 2018, I assume
> this is RHEL8 which does their own backports and never changes kernel version.
Indeed RHEL 8.9
> What is the kernel dmesg, why is it deciding on that value?
It comes from kernel's determine_cpu_tsc_frequencies() afaict (which
didn't change that much).
As a matter of fact, I got a similar behavior on my vmware VM on my
laptop (although smaller diff).
kernel: 4.18.0-553.8.1.el8_10.x86_64
lscpu:
Model name: 12th Gen Intel(R) Core(TM) i7-1260P
Stepping: 3
CPU MHz: 2495.994
BogoMIPS: 4991.98
Hypervisor vendor: VMware
dmesg | grep -i tsc
[ 0.000000] vmware: TSC freq read from hypervisor : 2495.994 MHz
[ 0.000000] tsc: Detected 2495.994 MHz processor
[ 0.000000] TSC deadline timer available
[ 0.010000] clocksource: tsc-early: mask: 0xffffffffffffffff
max_cycles: 0x23fa717cb36, max_idle_ns: 440795237972 ns
[ 0.905000] clocksource: Switched to clocksource tsc-early
[ 3.104381] tsc: Refined TSC clocksource calibration: 2495.990 MHz
[ 3.105473] clocksource: tsc: mask: 0xffffffffffffffff max_cycles:
0x23fa6db1dfc, max_idle_ns: 440795265852 ns
[ 3.264297] clocksource: Switched to clocksource tsc
cat /sys/devices/system/cpu/cpu0/tsc_freq_khz
2495990
sudo bpftrace -e 'BEGIN { printf("%u\n", *kaddr("tsc_khz")); exit(); }'
Attaching 1 probe...
2495990
My modified dpdk code above gives:
EAL: TSC frequency arch ~0 KHz
EAL: TSC frequency linux ~2495982 KHz
EAL: TSC frequency estimate ~2497263 KHz
Note that with the unmodified dpdk code which rounds to 10MHz both the
linux and the common estimation would give 2500000 KHz.
next prev parent reply other threads:[~2024-09-19 9:37 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-09-18 22:04 Isaac Boukris
2024-09-18 23:27 ` Stephen Hemminger
2024-09-19 9:37 ` Isaac Boukris [this message]
2024-09-19 12:26 ` Isaac Boukris
2024-09-19 13:04 ` Isaac Boukris
2024-09-19 18:33 ` Isaac Boukris
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