re: kern/57558: pgdaemon 100% busy - no scanning (ZFS case)

[matthew green <mrg%eterna23.net@localhost>]

just to clear up what i think is a confusion.

> My comments to your remark below - a longer (re-)explaination follows 
> after that.
>
> On 05/05/24 01:07, Chuck Silvers wrote:
> > On Thu, May 02, 2024 at 08:50:02AM +0000, Frank Kardel wrote:
> >>   Thus arc..c needs an additional check to determine needfree if the kva
> >>   free space falls below 10%.
> > The intention is that 64-bit kernels should configure enough KVA
> > to be able to map all of physical memory as kmem without running out of KVA.
> We are not really running out of KVA, this is not violated. We are 
> running into less than 10% free KVA available
> and ZFS still allocating KVA while other pools give up KVA which gets 
> allocated to ZFS UNTIL
> we fall below uvmexp.freetarg.

freetarg has nothing to do with KVA.  that's about free memory
(actual physical pages).  KVA is a space that gets allocated out
of, and sometimes that space has real pages backing it but not
always (and sometimes the same page may be mapped more the once.)
on 64-bit platforms, KVA is generally *huge*.

KVA starvation shouldn't happen -- we have many terabytes
available for the KVA on amd64, and pool reclaim only happens
when we run low on free pages.  KVA on amd64 already is large
enough to map all of physical memory in the 'direct map' region,
as well as other places as needed.

(it sounds like zfs needs to be able to reclaim pages like other
consumers?)


.mrg.