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Something that will also help, is setting your minimum IO unit to 4096 bytes, the Advanced Format sector size, because then your Direct IO system will just work, regardless of whether sysadmins swap disks of different sector sizes from underneath you.
I wrote this for Node.js, which is a native binding in C, exposing cross platform functionality: Īlthough if it's a new project and you're used to C, I would recommend also taking a good look at Zig ( ), because it's so explicit about alignment compared to C, and makes alignment a first-class part of the type system, see this other comment of mine that goes into more detail: īut how does it work for GPUs? Something to do with fixed pci-e support on the cpu (base address register )? (edit: this seems to say nothing similar is possible with mmap on x86. I guess mmapped files still may need a read to know whether to do copy on write, where mapped memory for the CPU in that case is specifically marked for upload only and gets something flagged that writes it regardless of if there is a change, but mmap maybe has something similar?
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For example, you can avoid the xor optimization by using a volatile pointer or by optimizing for code speed instead of code size.
> Use the appropriate optimization settings and language constructs to help avoid this performance penalty. > Even the following C++ code can read from memory and trigger the performance penalty because the code can expand to the following x86 assembly code. Isn't there a way around this? When coding for graphics stuff writing to GPU mapped memory people usually take pains to turn off compiler optimizations that might XOR memory against itself to zero it out or AND it against 0 and cause a read, and other things like that. It was at this point I understood memory alignment and why it might matter at a CPU level.
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"In-depth technical story: Fixing I/O performance for Windows guests in OpenStack Ceph clouds" It was made worse by Ceph behaviour that would write to that page, incur the penalty, immediately use madvise to tell the kernel I DONTNEED that anymore as an optimisation (because its a replica that is write-only, I don't need to read it again!), it got dropped, and a millisecond later wrote to it again and had to read it again (because that optimisation did not consider this case of unaligned writes needing to read) non 4kB-sized-and-aligned writes which happens from Windows guests (using 512b alignment, at least by default) but not Linux guests (which use 4kB alignment, even if the disk is 4kB aligned) It was caused by "unaligned" writes, e.g.
I debugged a Ceph issue being caused by this kind of behaviour except it wasn't actually mmap it was pwritev() but the call would stall and take a long time to return in that call that was expected to be async and go into the page cache. And then as you said, it stalls your write.
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It won't trigger a read if you write a full 4kB page in both size and alignment but does for anything else.
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