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1. The FreeBSD driver was setting an interrupt coalesce delay of 1000us for reasons that I can only speculate on. This was hurting everything from lame sequential I/O "benchmarks" to legitimate filesystem metadata operations that relied on serialized barrier writes. One of my filesystem tests went from 35s to complete down to 6s. 2. Implemented the Performant transport method. Without the fix in (1), I saw almost no difference. With it, my filesystem tests showed another 5-10% improvement in speed. It was hard to measure CPU utilization in any meaningful way, so it's not clear if there was a benefit there, though there should have been since the interrupt handler was reduced from 2 or more PCI reads down to 1. 3. Implemented MSI-X. Without any docs on this, I was just taking a guess, and it appears to only work with the Performant method. This could be a programming or understanding mistake on my part. While this by itself made almost no difference to performance since the Performant method already eliminated most of the synchronous reads over the PCI bus, it did allow the CISS hardware to stop sharing its interrupt with the USB hardware, which in turn allowed the driver to become decoupled from the Giant-locked USB driver stack. This increased performance by almost 20%. The MSI-X setup was done with 4 vectors allocated, but only 1 vector used since the performant method was told to only use 1 of 4 queues. Fiddling with this might make it work with the simpleq method, not sure. I did not implement MSI since I have no MSI-specific hardware in my test lab. 4. Improved the locking in the driver, trimmed some data structures. This didn't improve test times in any measurable way, but it does look like it gave a minor improvement to CPU usage when many processes/threads were doing I/O in parallel. Again, this was hard to accurately test.
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