Discussing emerging memory and Rust

[DrizztVD1]

Continuing the discussion from Rust beginner notes & questions:

Rust beginner notes & questions

Wouldn't it be more correct to say that you can mmap a 4GB window within a file larger than 4GB on the 32-bit platform? You can't actually map the whole file contiguously, right?

Rust beginner notes & questions

Yes. You can map up to a 2 GB view (actually several!) into a file of any size on Win32. The upper half of the address space is reserved for the kernel and cannot be used for mapping, even with the /3GB flag (apparently). I suspect Linux 32-bit has similar behaviour, but don't quote me on that. There's a diagram on this page: Memory-Mapped Files | Microsoft Docs To quote the KB article: "Multiple views may also be necessary if the file is greater than the size of the application’s logical …

Rust beginner notes & questions

I feel compelled to add that this was a thing in the 90's for Sun boxes, and was used to hide latency and coalesce IOPS to disk.

Rust beginner notes & questions

I suspect that was a battery-backed RAID controller cache, which is pretty common in all modern servers or SAN disk arrays. Compared to that, the future looks amazing: Intel Launches Optane DIMMs Up To 512GB: Apache Pass Is Here! This is just the beginning! Pretty soon you'll be seeing practically everyone running databases with 100% of the I/O coming directly from these. Compared to that, NVMe flash storage at a "mere" 3500 MB/s looks glacially slow!! Somewhat ironically, the great work tha…

Rust beginner notes & questions

Intel have made great claims about Optane before, and delivered in the end something which barely matches a modern SSD (and therefore does not justify the vendor lock-in or the software performance pitfall of adding yet another slower storage device to the virtual address space). I will wait for a more impressive product before calling this the future of storage. Moreover, asynchronous I/O is mostly used for network I/O, not disk I/O. Because for disk, we pay the price of yet another stupid leg…

Rust beginner notes & questions

No, it was host cpu memory, though it was mapped in the kernel with a device driver layer that used it exactly like RAID controllers would later.

[DrizztVD1]

@DrizztVD1

Somewhat ironically, the great work that has been done by the various C#, Java, JavaScript, and Rust core developers bringing asynchronous I/O programming to the masses is now hampering performance in this shiny new world of non-volatile memory. The overhead of even the most efficient futures-based async I/O is simply massive in comparison to the latency of NVDIMMs, which are byte-addressible and are only 2-3x slower than normal DIMMs. When your storage latency is measured in nanoseconds, individual instructions matter and user-to-kernel transitions are murder…

Here we see an opinion expressed by @peter_bertok on how Rust handles non-volatile (NV) memory. Discuss away.