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From: Stephen Hemminger <stephen@networkplumber.org>
To: Tal Shnaiderman <talshn@nvidia.com>
Cc: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>,
	"Dmitry Malloy (MESHCHANINOV)" <dmitrym@microsoft.com>,
	Narcisa Ana Maria Vasile <Narcisa.Vasile@microsoft.com>,
	Eilon Greenstein <eilong@nvidia.com>,
	Omar Cardona <ocardona@microsoft.com>,
	Rani Sharoni <ranish@nvidia.com>, Odi Assli <odia@nvidia.com>,
	Harini Ramakrishnan <Harini.Ramakrishnan@microsoft.com>,
	NBU-Contact-Thomas Monjalon <thomas@monjalon.net>,
	"dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] Windows DPDK real-time priority threads causing thread starvation
Date: Wed, 9 Dec 2020 08:08:58 -0800	[thread overview]
Message-ID: <20201209080858.168e4c52@hermes.local> (raw)
In-Reply-To: <DM6PR12MB39455643CEE4FF76DCA6743BA4CC0@DM6PR12MB3945.namprd12.prod.outlook.com>

On Wed, 9 Dec 2020 14:15:30 +0000
Tal Shnaiderman <talshn@nvidia.com> wrote:

> Hi,
> During our verification tests on Windows DPDK we've noticed that DPDK polling threads, which run in REALTIME_PRIORITY_CLASS are causing starvation to other threads from the OS which need to change affinity and run in lower priority.
> While running an application for a while we see the OS thread waits for 2:30 minutes and raises a bugcheck, see below example of such flow:
> 1) DPDK thread running on core-0 in real-time high priority(24) polling mode.
> 2) The thread is blocking the system function NtSetSystemInformation (ExpUpdateTimerConfiguration) in another thread from 
>    switching to core-0 via KeSetSystemGroupAffinityThread since the calling thread is priority 15. 
> 3) NtSetSystemInformation exclusively acquired system-wide lock (ExpTimeRefreshLock) hence 
>     it blocks other threads (e.g. calling NtQuerySystemInformation).
> We've seen this behavior only while running on Windows 2019 VMs, maybe on native machines OS scheduling of such flow is done differently? 
> Below is usage explanation from the documentation of SetPriorityClass [1]:
> Process that has the highest possible priority. The threads of the process preempt the threads of all other processes, including operating system processes performing important tasks. For example, a real-time process that executes for more than a very brief interval can cause disk caches not to flush or cause the mouse to be unresponsive. 
> So I assume using this kind of thread for a long period as we do can cause unstable behavior.
> How do you think we can resolve this? Are there such cases in Linux?
> [1] - https://docs.microsoft.com/en-us/windows/win32/api/processthreadsapi/nf-processthreadsapi-setpriorityclass
> Thanks,
> Tal.

This is not unique to Windows, Linux has same thing when using SCHED_FIFO.
Setting REALTIME is not a magic "go fast" flag it tells scheduler to "run this thread
at higher priority than kernel".  Setting real time is not compatible with applications
doing 100% polling. 

If you have to use REALTIME then application must change to doing sleep/wakeup type
architecture, not pure polling.

Typical DPDK style application is incompatible with SCHED_FIFO/SCHED_RR.

  parent reply	other threads:[~2020-12-09 16:09 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-09 14:15 Tal Shnaiderman
2020-12-09 16:08 ` John Alexander
2020-12-09 16:08 ` Stephen Hemminger [this message]
2020-12-09 16:12   ` [dpdk-dev] [EXTERNAL] " Dmitry Malloy (MESHCHANINOV)
2020-12-16 14:53     ` Tal Shnaiderman

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