rzezeski
Joined 15 years ago
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Member since
Mar 22
Recent submissions
Comment · 1 month ago
Comment · 2 years ago
I wrote a detailed walk-through of the illumos syscall handler back in 2016. It's missing the updates around KPTI (meltdown mitigation), but other than that the mechanisms should be unchanged.
Comment · 5 years ago
Probably more to do with the fact that everything is moving towards virtualization. Oftentimes these NICs dole out VFs directly to VMs via SR-IOV, in which case I imagine the NIC controller has some safeguards to keep the host and the rest of the guest's safe from denial-of-service and other attacks from a malicious guest driver.
Comment · 5 years ago
> During those tests, we noticed the machines were randomly freezing after some time, so we decided to upgrade the firmware of the network cards,
Reminds me of the various i40e Tx freezes I debugged while at Joyent. Granted, this is the illumos driver, not Intel's, but basically there were issues with the programming guide that I had to figure out the hard way. The 700-series controllers have not been the easiest to work with.
https://smartos.org/bugview/OS-7492 [Tx freeze when b_cont chain exceeds 8 descriptors]
https://smartos.org/bugview/OS-7457 [i40e Tx freezes on zero descriptors]
Comment · 7 years ago
You just figured it out.
Comment · 12 years ago
That's a good question. I should have mentioned that in the post perhaps. The reason I initially ran vmstat was just to make sure my dev machine was "zeroed out". Recently my kernel builds had taken twice the time as normal and I wasn't sure why. That's when I noticed this. It turns out this had no significant effect on my build times but was rather an effect of a different change I made weeks prior. I also noticed no ill effects on my laptop that I recall.
Comment · 12 years ago
According to POSIX.1b RT signals are used for other features defined in .1b. An excerpt from "POSIX.4 Programmers Guide":
> So far, we've talked about real-time signals sent by the sigqueue function. However, most POSIX.4 signals are sent as a result of three other facilities in POSIX.4. We'll cover these facilities in detail in their own chapters, but for now, you need to know that realtime signals can be generated as a result of expiration of a POSIX.4 timer, completion of asynchronous I/O, and by arrival of a message on an empty message queue.
Linux reserves 2-3 RT signals for use in its threading library:
Comment · 12 years ago
All kernels do not use the same libc. In fact, Linux is different in the fact that it doesn't ship a libc with the kernel. Illumos, the OS I investigated, goes by the philosophy that you _have_ to ship libc with your kernel, and thus they have their own libc.
My diagnosis for Illumos may not hold up on other OSs, they may invert priority for other reasons. In any case the spec is fairly clear that a signal is considered delivered when the "appropriate action" is taken. In this case the appropriate action is to invoke the user's handler, which is above libc on the stack.
Finally, in case you missed it, I witnessed 4 unique orderings across the systems I tested. They were not the same across the board.
Comment · 12 years ago
Author here, I won't get to it today but I would like to try the method you describe. I realize that benchmarking is a very tricky thing having followed Brendan Gregg's work the last couple of years. This is why I also ran my baseline benchmark which simply performed a ret. I learned years ago from tuning cars that absolute numbers are less interesting than relative. That said, I am a total newb at low-level stuff like this and I would not be surprised if I messed something up. I'll be curious to use your method and see how it compares.
Comment · 12 years ago
Hi, author here. My align_2 implementation is indeed very naive. Part of that was to show just how much overhead a naive implementation could add, in this case 3 orders of magnitude. I'm also new to systems programming. This article should serve as a reminder to experienced hackers that idioms are often only obvious _after_ they've been explained. My last 4 years were spent working on a distributed database; I could bring up "obvious" idioms from that world that might perplex a kernel hacker.
But you are right, this is indeed a very obvious line of code once you understand it. Thank you for your comments.
We are not the same.