Re: [dpdk-dev] [PATCH v16 2/9] eal: add thread attributes
From: Stephen Hemminger <stephen@networkplumber.org>
Date: 2021-11-10 03:59:38
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. 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.