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Mesa 22.0 Delays and LWN's Kernel Articles

Submitted by Roy Schestowitz on Thursday 13th of January 2022 10:11:08 AM Filed under
Linux
  • Mesa 22.0 Pushed Back By Three Weeks - Phoronix

    While a lot of open-source OpenGL and Vulkan driver improvements have been landing in recent days in anticipation of the Mesa 22.0 code branching and feature freeze for Wednesday, that deadline has now been extended by three weeks.

    Due to problems merging some merge requests from GitLab as well as FreeDesktop.org hosting issues, Mesa 22.0 has been pushed back. Additionally, some Mesa3D developers have expressed interest in trying to squeeze in some remaining patches not yet merged.

  • Zero-copy network transmission with io_uring [LWN.net]

    When the goal is to push bits over the network as fast as the hardware can go, any overhead hurts. The cost of copying data to be transmitted from user space into the kernel can be especially painful; it adds latency, takes valuable CPU time, and can be hard on cache performance. So it is unsurprising that the developers working with io_uring, which is all about performance, have turned their attention to zero-copy network transmission. This patch set from Pavel Begunkov, now in its second revision, looks to be significantly faster than the MSG_ZEROCOPY option supported by current kernels.

    As a reminder: io_uring is a relatively new API for asynchronous I/O (and related operations); it was first merged less than three years ago. User space sets up a pair of circular buffers shared with the kernel; the first buffer is used to submit operations to the kernel, while the second receives the results when operations complete. A suitably busy process that keeps the submission ring full can perform an indefinite number of operations without needing to make any system calls, which clearly improves performance. Io_uring also implements the concept of "fixed" buffers and files; these are held open, mapped, and ready for I/O within the kernel, saving the setup and teardown overhead that is otherwise incurred by every operation. It all adds up to a significantly faster way for I/O-intensive applications to work.

    One thing that io_uring still does not have is zero-copy networking, even though the networking subsystem supports zero-copy operation via the MSG_ZEROCOPY socket option. In theory, adding that support is simply a matter of wiring up the integration between the two subsystems. In practice, naturally, there are a few more details to deal with.

    A zero-copy networking implementation must have a way to inform applications when any given operation is truly complete; the application cannot reuse a buffer containing data to be transmitted if the kernel is still working on it. There is a subtle point that is relevant here: the completion of a send() call (for example) does not imply that the associated buffer is no longer in use. The operation "completes" when the data has been accepted into the networking subsystem for transmission; the higher layers may well be done with it, but the buffer itself may still be sitting in a network interface's transmission queue. A zero-copy operation is only truly done with its data buffers when the hardware has done its work — and, for many protocols, when the remote peer has acknowledged receipt of the data. That can happen long after the operation that initiated the transfer has completed.

    So there needs to be a mechanism by which the kernel can tell applications that a given buffer can be reused. MSG_ZEROCOPY handles this by returning notifications via the error queue associated with the socket — a bit awkward, but it works. Io_uring, instead, already has a completion-notification mechanism in place, so the "really complete" notifications fit in naturally. But there are still a few complications resulting from the need to accurately tell an application which buffers can be reused.

  • User-managed concurrency groups [LWN.net]

    The kernel's thread model is relatively straightforward and performs reasonably well, but that's not enough for all users. Specifically, there are use cases out there that benefit from a lightweight threading model that gives user space control over scheduling decisions. Back in May 2021, Peter Oskolkov posted a patch set implementing an abstraction known as user-managed concurrency groups, or UMCG. Several revisions later, many observers still lack a clear idea of what this patch is supposed to do, much less whether it is a good idea for the kernel. Things have taken a turn, though, with Peter Zijlstra's reimplementation of UMCG.
    One developer reimplementing another's patch set is likely to raise eyebrows. Zijlstra's motivation for doing that work can perhaps be seen in this message, where he notes that the UMCG code looked little like the rest of the scheduler code. He also remarked that it required "reverse engineering" to figure out how UMCG was meant to be used. By the time that work was done, perhaps, it was just easier to recast the code in the form he thought it should take.

    In truth, the documentation for UMCG is no better than before — a significant problem for a major proposed addition to the system-call API. But it is possible to dig through the code (and a "pretty rough" test application posted by Zijlstra) to get a sense for what is going on. In short, UMCG calls for a multi-threaded application to divide itself into "server" and "worker" threads, where there is likely to be one server thread for each CPU on the system. Server threads make scheduling decisions, while workers run according to those decisions and get the actual work done. The advantage of a system like UMCG is that scheduling can happen quickly and with little overhead from the kernel — assuming the server threads are properly implemented, of course.

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