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Cake day: June 2nd, 2023

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  • In my experience, nouveau is painfully slow and crashes constantly to the point of being virtually unusable for anything. The developers agree, as in the last couple months nouveau has been phased out of Mesa entirely. More recent Mesa versions now implement OpenGL on Nvidia using Zink on NVK, and the result is quite a bit faster and FAR more stable.

    If your distribution currently still ships a Mesa version which uses nouveau, I would personally recommend you just stick with the Intel graphics for now.


  • Aside from checking the kernel log (sudo dmesg) and system log (sudo journalctl -xe) for any interesting messages, I might suggest simply watching for any processes which are abnormally high while the system is running slow. My initial approach would be to use htop (disable “Hide Kernel Threads” and enable “Detailed CPU Time”), and seeing which processes, if any, are eating up your CPU time. The colored core utilization bars at the top show how much CPU time is being spent on what: gray for disk wait, red for kernel, green for regular user process, etc. That information will be a good starting point.






  • Roscoe is one of my professors at ETH, and he gave a keynote at VISCon a few months ago where he discussed this stuff and what his department is working on. Apparently a lot of their (they being the systems department at ETH) current work is related to formally modeling which parts of a system have access to what other parts, and then figuring out which of those permissions are actually needed and then deriving the strictest possible MPU configuration while still having a working system. The advantage of this approach over an entirely new kernel is that, well, it doesn’t require an entirely new kernel, but can be built into an existing system, while still allowing them to basically eliminate the entire class of vulnerabilities they’re targeting.


  • This guy (Roscoe) is one of my professors and I’ve heard him give a few talks related to this before, so I’ll try to summarize the problem:

    Basically, modern systems do not really match with the classic model of “there’s a some memory and perhipheral devices attached to a bus, and they’re all driven by the CPU running a kernel which is responsible for controlling everything”. Practically every component has it’s own memory and processor(s), each running their own software independently of the main kernel (sometime even with their own separate kernel!), there are separate buses completely inaccessible to the CPU specifically for communicating between components, often virtually every component is directly attached to the memory bus and therefore bypasses the CPU’s memory protection mechanisms, and a lot of these hidden coprocessors are completely undocumented. A modern smartphone SoC can have 10s of separate processors all running their own software independently of each other.

    This is bad for a lot of reasons, most importantly that it becomes basically impossible to reason about the correctness or security of the system when the “OS kernel” is actually just one of many equally privileged devices sharing the same bus. An example of what this allows: it is (or was) possible to send malformed WiFi packets and trigger a buffer overrun in certain mobile WiFi modems, allowing an attacker to get arbitrary code execution on the modem and then use that to overwrite the linux kernel in main memory, thus achieving full kernel-level RCE with no user interaction required. You can have the most security-hardened linux kernel you want, but that doesn’t mean a damn thing if any one of dozens of other processors can just… overwrite your code or read sensitive data directly from applications!

    As I understand it, the goal of these projects is basically to make the kernel truly control all the hardware again, by having them also provide the firmware/control software for every component in the system. Obviously this requires a very different approach than conventional kernel designs, which basically just assume they rule the machine.


  • This is specific to page reclamations, which only occur when the kernel is removing a block of memory from a process. VMs in particular pretty much never do this; they pin a whole ton of memory and leave it entirely up to the guest OS to manage. The JVM also rarely ever returns heap memory to the kernel - only a few garbage collectors even support doing so (and support is relatively recent), and even if you have it configured correctly it’ll only release memory when the Java application is relatively idle (so the performance hit isn’t noticeable).



  • This probably won’t make much difference unless your application is frequently adding and removing large numbers of page mappings (either because it’s explicitly unmapping memory segments, or because pages are constantly being swapped in and out due to low system memory). I would suspect that the only things which would really benefit from this under “normal” circumstances are some particularly I/O intensive applications with lots of large memory mappings (e.g. some webservers, some BitTorrent clients), or applications which are frequently allocating and deallocating huge slabs of memory.

    There might be some improvement during application startup as all the code+data pages are mapped in and the memory allocator’s arenas fill up, but as far as I know anonymous mappings are typically filled in one page at a time on first write so I don’t know how relevant this sort of batching might be.



  • Traditional graphics code works by having the CPU generate a sequence of commands which are packed together and sent to the GPU to run. This extension let’s you write code which runs on the GPU to generate commands, and then execute those same commands on the GPU without involving the CPU at all.

    This is a super powerful feature which makes it possible to do things which simply weren’t feasible in the traditional model. Vulkan improved on OpenGL by allowing people to build command buffers on multiple threads, and also re-use existing command buffers, but GPU pipelines are getting so wide that scenes containing many objects with different render settings are bottlenecked by the rate at which the CPU can prepare commands, not by GPU throughput. Letting the GPU generate its own commands means you can leverage the GPU’s massive parallelism for the entire render process, and can also make render state changes much cheaper.

    (For anyone familiar, this is basically a more fleshed out version of NVIDIA’s proprietary NV_command_list extension for OpenGL, except that it’s in Vulkan and standardized across all GPU drivers)







  • It’s not that obscure - I had a use case a while back where I had multiple rocksdb instances running on the same machine and wanted each of them to store their WAL only on SSD storage with compression and have the main tables be stored uncompressed on an HDD array with write-through SSD cache (ideally using the same set of SSDs for cost). I eventually did it, but it required partitioning the SSDs in half, using one half for a bcache (not bcachefs) in front of the HDDs and then using the other half of the SSDs to create a compressed filesystem which I then created subdirectories on and bind mounted each into the corresponding rocksdb database.

    Yes, it works, but it’s also ugly as sin and the SSD allocation between the cache and the WAL storage is also fixed (I’d like to use as much space as possible for caching). This would be just a few simple commands using bcachefs, and would also be completely transparent once configured (no messing around with dozens of fstab entries or bind mounts).