Linux 4.6 is out

I'd also dispute the suggestion that Plan9 is more advanced than Linux. It's different and Plan9 definitely does some interesting things, but that doesn't automatically make it more advanced. Ignoring that "advanced" is hard to quantify, I'd more that Plan9 is simply based on different concepts. It's like comparing Windows and Linux, albeit maybe not quite so extreme as Plan9 and Linux have more in common and Plan9 is simply not as heavily developed as either OS.

Linux uses greedy caching for filesystems and it's usually pretty awesome. But by merely opting in to be in a memory-constrained cgroup, it is (was?) no longer able to reap those pages in the background. Perhaps it's because the kernel daemons that would ordinarily do that don't have the right context. Or in order to have it they'd need to have one instance for each cgroup dir/context.

Effectively what we found was that opting out of cgroups memory constraints meant that we would still cause the filesystem cache to get evicted in order to make room for our memory allocations but with significantly lower latency. I suspect that what might have been happening is that when cgroup-memory-bound, we faulted in a disk-backed page and that triggered logic that decided it needed to evict other pages in order to make room for this one. So far, so good. But then perhaps it got opportunistic and figured "since I can't know that I need to background-evict these pages I had better foreground-evict as many as I can now." This is all wild speculation on my part. But what was clear was that our process would suffer a multi-second stall while evicting pages. Disabling cgroups-memory constraints meant that we would not hit this same condition.

More background: our application would read mapped files into memory, transform the data then write it out over sockets. The file-reading-task would read files whose size exceeds the total system memory. All files were opened read-only, the pages were never dirtied. There were no swap devices.

I don't recall all the specifics but the gist we got from the distro was that "yeah, it works that way and it's not a bug."

All I wanted to do was make it so that the greedy process could eat up no more than half of system memory with stuff like the fscache or dumb allocations. That would mean that other processes would be much less likely to stall when doing memory allocations.

Watch the dirty cache data when this is happening, if you want to know for sure. Put a couple "sync;sync;sync"'s &&'ed on that cat, if you want to watch stimulating behaviour..

(More often than not, its the device. Linux is dealing with it fine.)

(ducks)

:-)

You go on to praise Windows, as if it's okay to ignore history when it comes to MS.

Eh... I think people in HN get overly offended when people state the fact that Linux has been historically revolutionary.

Kind of cool how randomness creates forks in time.

  Yeah but *BSD isn't nearly as wide
  spread as Linux.