Defaulting To 4K Stacks

Jeremy — Wed, 23 Apr 2008 01:42:24 +0000

Andrew Morton replied to a commit message making 4k stacks the default, saying, "this patch will cause kernels to crash." Ingo Molnar replied, "what mainline kernels crash and how will they crash? Fedora and other distros have had 4K stacks enabled for years." He added, "we've conducted tens of thousands of bootup tests with all sorts of drivers and kernel options enabled and have yet to see a single crash due to 4K stacks." During the lengthy discussion it was suggested that nfs+xfs+raid kernel configurations, and using ndiswrapper are the most common reasons for overflowing a 4K stack size.

Andi Kleen questioned the usefulness of 4k stacks, "as far as I can figure out they are not [a worthy goal]. They might have been a worthy goal on crappy 2.4 VMs, but these times are long gone." Arjan van de Ven suggested that though the 2.6 VM is much improved over the 2.4 VM, fragmentation with 8K stacks remains an unsolvable problem, "it's basic math; the Linux VM gets to deal with both short and long lasting allocations; no matter how hard you try to get some degree of fragmentation; especially due to the 15:1 acceleration you get due to the lowmem issue. And before you say 'you should use 64 bit on such machines'; I would love it if more people used 64 bit linux. Sadly the adoption rate of that is not very good still.... by far ;(" In another email, Arjan listed two advantages to 4K stacks, "1) less memory consumption in the lowmem zone (critical for enterprise use, also good for general performance), and 2) kernel stacks at 8K are one of the most prominent order-1 allocations in the kernel; again with big-memory systems the fragmentation of the lowmem zone is a problem (and the distros that ship 4K stacks went there because of customer complaints)".

Linux: Moving 4K Stacks Forward

Jeremy — Thu, 16 Aug 2007 20:42:16 +0000

In a series of 5 patches, Jesper Juhl propsed moving 4K stacks from a debug feature to a non-debug feature, defaulting it to be enabled in the -mm tree. He referred back to a lengthy earlier discussion in which he had proposed making 4K stacks the default in the mainline kernel, then added:

"Based on the comments in that thread I conclude that 4KSTACKS are not really considered a debug-only feature any longer, but the time is not right (yet) to make them the default - and it's certainly not yet the time to get rid of 8K stacks."

"In that thread I promised to provide some patches that would lift 4KSTACKS out of debug-only feature status, which is what the first two patches in this series do. I also said I would provide a patch to make 4KSTACKS 'default y' to get more testing, but restrict that patch to -mm - that's the fifth patch in this series. Patches 3 & 4 in this series move the config option out of the Kernel hacking menu and into Processor types and features".

KernelTrap - 4k stacks

Defaulting To 4K Stacks

Linux: Moving 4K Stacks Forward