Re: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19

Linus Torvalds
2005-11-04T16:07:47Z

On Fri, 4 Nov 2005, Andy Nelson wrote:
>
> AFAIK, mips chips have a software TLB refill that takes 1000
> cycles more or less. I could be wrong.

You're not far off.

Time it on a real machine some day. On a modern x86, you will fill a TLB
entry in anything from 1-8 cycles if it's in L1, and add a couple of dozen
cycles for L2.

In fact, the L1 TLB miss can often be hidden by the OoO engine.

Now, do the math. Your "3-4 time slowdown" with several hundred cycle TLB
miss just GOES AWAY with real hardware. Yes, you'll still see slowdowns,
but they won't be nearly as noticeable. And having a simpler and more
efficient kernel will actually make _up_ for them in many cases. For
example, you can do all your calculations on idle workstations that don't
mysteriously just crash because somebody was also doing something else on
them.

Face it. MIPS sucks. It was clean, but it didn't perform very well. SGI
doesn't sell those things very actively these days, do they?

So don't blame Linux. Don't make sweeping statements based on hardware
situations that just aren't relevant any more.

If you ever see a machine again that has a huge TLB slowdown, let the
machine vendor know, and then SWITCH VENDORS. Linux will work on sane
machines too.

Linus