RE: [EXTERNAL] Re: [dpdk-dev] [PATCH v16 2/9] eal: add thread attributes

-----Original Message-----
From: Stephen Hemminger <stephen@networkplumber.org> 
Sent: Tuesday, November 9, 2021 8:00 PM
To: Narcisa Ana Maria Vasile <redacted>
Cc: thomas <redacted>; dev@dpdk.org; Dmitry Kozlyuk <redacted>; Khoa To <redacted>; Dmitry Malloy <redacted>; Tyler Retzlaff <redacted>; talshn <redacted>; Omar Cardona <redacted>; bruce.richardson@intel.com; david.marchand@redhat.com; Kadam, Pallavi <redacted>
Subject: [EXTERNAL] Re: [dpdk-dev] [PATCH v16 2/9] eal: add thread attributes

On Tue, 9 Nov 2021 19:04:17 -0800
Narcisa Ana Maria Vasile [off-list ref] wrote:

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  I'll send a new version with a better commit message.
  Thread priorities on both Linux-based and Windows platforms are similarly
  constructed from a class/policy + priority value. Currently in DPDK, most threads
  operate at the OS-default priority level but there are cases when increasing the
  priority is useful. For example, the Mellanox data path acceleration driver requires
  realtime thread priority. Similarly, some Windows applications will require elevated
  priority.  

 It should not. We should not use realtime priority.  

Thomas, can you join the community sync tomorrow? I'll bring this up to discuss.

High performance applications benefit from an option to raise the 
priority of their threads to avoid being preemted by other threads on 
the system. If there are issues with realtime priority on some of the 
platforms, maybe we can add a warning for the user to make them aware 
of possible crashes as Stephen H. suggested some time ago. Note that 
this patch doesn't change the priority of EAL threads, enabling the higher priority will be done through a command line option when starting the application.
Maybe we can explore raising the priority but not to the realtime level.

Let me put it more succulently.
Almost all DPDK applications have threads that are a 100% CPU doing polling.
Putting those thread as real-time thread breaks Linux badly because the kernel can and will try and run work on those CPU's and the system is broken/unstable/dead at that point.

The suggestion is that when running on Windows it should be possible for the application to be configured to use real-time threads, there is no implication that it will force real-time priority on Linux.  If it doesn't make sense for it to be configured on Linux then don't configure it.  But saying it shall not be configurable for any platform just because one platform can't make use of the configuration and those platforms have to run compromised makes little sense.  Linux administrators are (or should be) knowledgeable enough to know what configuration to use.

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This is a case of different definitions and expectations of real-time in Linux and Windows.  Linux definition of real-time priority is for well behaved and time critical applications. It expects RT applications to run, then sleep.