Messages in this thread |  | | Date | Mon, 30 Oct 2000 09:16:15 +0100 | From | Andi Kleen <> | Subject | Re: 2.2.18Pre Lan Performance Rocks! |
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On Mon, Oct 30, 2000 at 01:04:34AM -0700, Jeff V. Merkey wrote: > > > > > > When you say it reloads it's VM, you mean it reloads the CR3 register? > > > > Yes. > > > > No. In 2.4 you could probably use the on demand lazy vm mechanism ingo described for > > the nfsd processes. In 2.2 it is a bit more tricky, if I remember right lazy mm needed > > quite a few changes. > > > > But before doing too many changes i would first verify if that is really the problem. > > We will never beat NetWare on scaling if this is the case, even in 2.4. > Andre and my first job will be to create an arch port with MANOS that > disables this and restructures the VM.
I just guess the end result will be as crash prone as Netware when you install any third party software ;)
lazy mm is probably a better path, as long as you stay in kernel threads and a single user mm it'll never switch VMs.
> > > PTEs are read for aging on memory pressure in vmscan. > > > > segment register reload happen on interrupt entry, to load the kernel cs/ds (are they that > > costly?). If it was really costly you could probably check in interrupt entry if you're > > already running in kernel space and skip it. > > They are. segment register reloads will trigger the following: > > IDT table atomic fetch to verify (LOCK#) (if triggered by task gate from INTR) > GDT table atomic fetch to verify (LOCK#) > LDT table atomic fetch to verify (LOCK#) (if present) > PDE table atomic fetch to verify (LOCK#) > > The process has to verify that the loaded segment descriptor is valid, and > it will fetch from all these tables to do it, with up to 4 (LOCK#) > assertions occurring invisibly in the hardware underneath (which will > generate 4 non-cacheable memory references, in addition to wrecking > havoc on the affected L1/L2 cache lines). Oink. It only does this > when you load one, not when you save one, like pushing it on the > stack. Since you look at MANOS code, you'll note that in > CONTEXT.386, I do and add esp, 3 * 4 instead of poppping the segment > registers off the stack if they are in the kernel address space. > > Linux should do the same, if possible as an optimization.
Interesting. You could do easily the same in Linux by changing (in 2.2) the SAVE_ALL macro in arch/i386/kernel/irq.h and doing the same in ret_from_intr's RESTORE_ALL macro (after changing it to a RESTORE_ALL_INT or you'll break system calls)
Actually I don't even know why irq.h's SAVE_ALL even loads __KERNEL_CS, it should be already set by the interrupt gate. __KERNEL_DS probably needs to be still loaded, but only when you came from user space.
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