Messages in this thread | | | Date | Mon, 29 Apr 2013 15:08:50 -0700 | From | "Pierre-Loup A. Griffais" <> | Subject | Re: IO regression after ab8fabd46f on x86 kernels with high memory |
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On 04/29/2013 03:03 PM, Linus Torvalds wrote: > On Mon, Apr 29, 2013 at 2:53 PM, Pierre-Loup A. Griffais > <pgriffais@valvesoftware.com> wrote: >> >> Other than this particular concern, what's the high-level take-away? Is PAE >> support in the Linux kernel a false promise than distros should not be >> shipping by default, if at all? Should it be removed from the kernel >> entirely if these configurations are knowingly broken by commits like this? > > PAE is "make it barely work". The whole concept is fundamentally > flawed, and anybody who runs a 32-bit kernel with 16GB or RAM doesn't > even understand *how* flawed and stupid that is. > > Don't do it. Upgrade to 64-bit, or live with the fact that IO > performance will suck. The fact that it happened to work better under > your particular load with one particular IO size is entirely just > "random noise". > > Yeah, the difference between "we can cache it" and "we have to do IO" > is huge. With a 32-bit kernel, we do IO much earlier now, just to > avoid some really nasty situations. That makes you go from the "can > sit in the cache" to the "do lots of IO" situation. Tough. > > Seriously, you can compile yourself a 64-bit kernel and continue to > use your 32-bit user-land. And you can complain to whatever distro you > used that it didn't do that in the first place. But we're not going to > bother with trying to tune PAE for some particular load. It's just not > worth it to anybody.
All of this came from me trying to reproduce slowdowns reported by other people; I personally run a 64-bit kernel and understand how bad of an idea it is to attempt to run 32-bit kernels with PAE enabled on modern machines. However, my goal is to avoid ending up with a variety of end-users that don't necessarily understand this getting bitten by it and breaking their systems by upgrading their kernels. I will indeed bring this up with distributors and point out than shipping PAE kernels by default is not a good idea given these problems and your stance on the matter.
Thanks, - Pierre-Loup
> > Linus >
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