Re: [PATCH] x86: have set_memory_array_{uc,wb} coalesce memtypes.

[Rene Herman]

On 22-08-08 21:08, Venki Pallipadi wrote:

> On Thu, Aug 21, 2008 at 09:15:44PM -0700, Ingo Molnar wrote:

> > Venki, Suresh, Shaohua, Dave, Arjan - any observations about this
> > line of action?
>
> The concern I have here is that the coalescing is not guaranteed to
> work. We may still end up having horrible worst case latency, even
> though this improves the normal case (boot the system, start X, exit
> X, reboot the system). It depends on how pages are allocated and how
> much memory is there in the system and what else is running etc.

Yes, I agree. Independent of the current trigger PAT wants a more 
scalable approach and yes, worst case is still single page entries.

That worst case is the guaranteed case now though, so I do feel it's a 
generic fix. After all, there wouldn't seem to be a reason to _not_ 
coalesce in set_memory_array_{uc,wb}().

> Here on my test system, without this coalescing change I see
>
> [root@localhost ~]# cat /proc/sys/debug/x86/pat_memtype_list | wc -l
> 19528
>
> With the coalescing change I see
> [root@localhost ~]# cat /proc/sys/debug/x86/pat_memtype_list | wc -l
> 135
>
> quit and restart X
> [root@localhost ~]# cat /proc/sys/debug/x86/pat_memtype_list | wc -l
> 985

[ constantly growing number of entries ]

Yes, absolutely right, PAT definitely needs something other than the 
simple linked list. I do believe we also want the coalescing change 
though - it seems to make sense regardless of trigger and it's only 
little code.

> I think this as a good workaround for now. But, for long run we still need to
> look at other ways of eliminating this overhead (like using page struct
> that Suresh mentioned in the other thread).
>
>
> Also, there seems to be a bug in the error path of the patch. Below should
> fix it.

Ah, yes, thanks, just sent out a final version with this fixed as well.

Rene.

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