Score:0

PCIe readout performance depending on kernel

fr flag

I have a PCIe (Gen3) device that transfers data to the computer RAM in chunks of 256 MiB, each copy taking about 48 ms. I am using Ubuntu 20.04 and a kernel device module to handle the data transfer. Whenever I upgrade to a newer kernel, I recompile the driver.

I recently noticed that with image 5.8.0-53, the readout was consistent, and taking about 48ms (+/- 0.2 ms). After upgrading to 5.8.0-55 (same for -57 and -59), the operation takes 51 ms. The increase is not convenient as it's a real time application and it should stay at or below 50ms in order not to lose data.

I was curious to know what might be the reason behind and if there is a changelog pointing at what changes might have caused this change, or if there is any way to give feedback via GitHub? Maybe it's a regression and newer kernel versions have better performance? Thanks in advance for your advice.

Doug Smythies avatar
gn flag
It can be very difficult and time consuming to isolate the exact commit that introduced your change in performance. As a test, I would try the latest mainline kernel, currently [5.13](https://kernel.ubuntu.com/~kernel-ppa/mainline/v5.13/) to determine if your issue persists upstream. If it does, I would try other mainline kernels in an attempt to determine the start and end points for a kernel bisection. Then bisect to isolate the problematic commit. There is likely also a away to bisect between 5.8.0-53 and 5.8.0-55, but that would be a workflow I am not familiar with.
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