Strange ARC/Swap/CPU on yesterday's -CURRENT

[Bryan Drewery]

On 3/20/2018 12:07 AM, Peter Jeremy wrote:
> 
> On 2018-Mar-11 10:43:58 -1000, Jeff Roberson <jroberson at jroberson.net> wrote:
>> Also, if you could try going back to r328953 or r326346 and let me know if 
>> the problem exists in either.  That would be very helpful.  If anyone is 
>> willing to debug this with me contact me directly and I will send some 
>> test patches or debugging info after you have done the above steps.
> 
> I ran into this on 11-stable and tracked it to r326619 (MFC of r325851).
> I initially got around the problem by reverting that commit but either
> it or something very similar is still present in 11-stable r331053.
> 
> I've seen it in my main server (32GB RAM) but haven't managed to reproduce
> it in smaller VBox guests - one difficulty I faced was artificially filling
> ARC.
> 

Looking at the ARC change you referred to from r325851
https://reviews.freebsd.org/D12163, I am convinced that ARC backpressure
is completely broken. On my 78GB RAM system with ARC limited to 40GB and
doing a poudriere build of all LLVM and GCC packages at once in tmpfs I
can get swap up near 50GB and yet the ARC remains at 40GB through it
all.  It's always been slow to give up memory for package builds but it
really seems broken right now.

-- 
Regards,
Bryan Drewery

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