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Date: Wed, 1 May 2013 21:37:27 -0700 From: Sonny Rao <sonnyrao@...omium.org> To: "Pierre-Loup A. Griffais" <pgriffais@...vesoftware.com> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Johannes Weiner <hannes@...xchg.org>, Rik van Riel <riel@...hat.com>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>, Andrew Morton <akpm@...ux-foundation.org> Subject: Re: IO regression after ab8fabd46f on x86 kernels with high memory On Mon, Apr 29, 2013 at 3:08 PM, Pierre-Loup A. Griffais <pgriffais@...vesoftware.com> wrote: > On 04/29/2013 03:03 PM, Linus Torvalds wrote: >> >> On Mon, Apr 29, 2013 at 2:53 PM, Pierre-Loup A. Griffais >> <pgriffais@...vesoftware.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. > Sorry just saw this (my stupid gmail filters for lkml) The slow-down we ran into wasn't even on PAE -- it was *just* with highmem on a 2GB system. The non-zero amount (90MB? or so) of highmem was enough to cause major problems due to that particular underflow. I would say regardless of how much memory you have, if the system can use a 64-bit kernel, then it almost certainly should. I've seen some very minor performance impacts on 64-bit capable Atom systems with tiny L2 caches, but it's almost in the noise and not worth the pain. > Thanks, > - Pierre-Loup > >> >> Linus >> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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