Unmapped I/O improves userland direct disk performance by 35% ~ 135% (opens in new tab)

Yeah, the cloud is finally catching up with the mainframes.

Hyper-v kinda sucked with FreeBSD, mostly just overall poor performance. If you needed FreeBSD that would dictate what you ran it on, so hyper-v would be out. If you had to run hyper-v, FreeBSD would be out. Many enterprises find Windows to be mandatory, even if it's just running virtual machines, even if a better solution is out there. UNIX is often replaceable because "they're all the same."

We updated the title from ”Sponsored by Microsoft – Improves userland disk I/O performance by 35% – 135%” which was editorialized.

For those wondering what is going on, the original submission title started with "sponsored by Microsoft". It has since been edited.

Diff tools that can ignore white space changes make code reviews a lot easier.

I'm looking on a phone, so maybe I'm missing it, but I don't see much gratuitous whitespace change. The only place I see that is in the declaration of hv_storvsc_request.

Indeed, Microsoft has done quite a bit of work towards increasing the performance of FreeBSD running on Azure / HyperV.

Yes, they're doing this with Linux too. I recall reading an announcement about 20kLoC worth of their patches to the Linux mainline kernel, in order to improve performance on Hyper-V.

I had the same question, or maybe even a more basic one: what do they mean by "unmapped I/O", and is there a different term for this concept in Linux? Does it perhaps relate to O_DIRECT? To sendfile() and splice()? This paper is the closest to an answer that I've found: http://www.cs.technion.ac.il/~dan/papers/cim-asplos-2016.pdf

I think the point is that, currently, BSD (at least with Hyper-V) defaults to a "mapped" zero-copy approach: when a read is requested, the user space buffer is mapped to a DMA accessible address, the device DMA's the data directly into the buffer, and then the DMA address is unmapped.

Unfortunately, the IOMMU operations to make this happen are not free, and have side-effects like a TLB flush. For small transfers, it's cheaper to let the DMA write to a persistent kernel buffer, and then have the kernel copy it to user-space. I think this traditional non-zero-copy approach is what they are referring to as "unmapped".

But I'm guessing. It would be kind if someone fluent in BSD could explain it correctly.

FreeBSD running on Hyper-V (Microsoft's virtualization layer) was slow; now it's faster thanks to code Microsoft contributed (EDIT: or simply sponsored, it's not clear).