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How does this work? I don't see how its possible to fight the scheduler or how caching works via userspace. Maybe I don't understand this stuff, but maybe the

• dap 13y ago
  You can bypass the scheduler by binding processes to processors. Some runtimes do this -- and sometimes get much worse performance as a result. My colleagues and I have seen many programs that think they know better than the OS do much worse for trying.
• usefulcat 13y ago
  You can bypass filesystem caching on Linux using O_DIRECT. The tradeoff (besides the obvious lack of caching) is that there are specific alignment restrictions on the length and address of the userspace buffers and file offsets, and these restrictions may vary by filesystem and by kernel.
• jandrewrogers 13y ago
  The kernel does have bypass APIs, or APIs that can be used for that purpose. However, you can't use just a little; once you start down that path you need to bypass everything. To be clear, while the bypass APIs are simple to use you actually have to know what you are doing since you become responsible for doing things the OS used to do for you. I/O scheduling, disk caching, process scheduling, memory management, etc all have to be reimplemented in userspace. It is why I mentioned that the skill set required to do bypass kernels is fairly rarified. You can't just reimplement what the OS already does, you need to implement something that is different than the OS design but also better at providing functionality the OS provides for the use case. You are essentially writing a purpose-optimized OS without the device drivers.