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

["Chuck Silvers via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR kern/57558; it has been noted by GNATS.

From: Chuck Silvers <chuq%chuq.com@localhost>
To: Frank Kardel <kardel%netbsd.org@localhost>
Cc: gnats-bugs%netbsd.org@localhost, kern-bug-people%netbsd.org@localhost,
	netbsd-bugs%netbsd.org@localhost
Subject: Re: kern/57558: pgdaemon 100% busy - no scanning (ZFS case)
Date: Sat, 7 Dec 2024 16:20:02 -0800

 On Thu, May 09, 2024 at 09:06:43PM +0200, Frank Kardel wrote:
 > On 05/09/24 19:48, Chuck Silvers wrote:
 > 
 > > great, thanks for testing.
 > > 
 > > which other cases are you talking about?
 > > 
 > > -Chuck
 > 
 > The cases in the KVA nkmempages determination.
 > 
 > #if defined(NKMEMPAGES_MAX_UNLIMITED) && !defined(KMSAN)
 >         /*
 >          * The extra 1/9 here is to account for uvm_km_va_starved_p()
 >          * wanting to keep 10% of kmem virtual space free.
 >          * The intent is that on "unlimited" platforms we should be able
 >          * to allocate all of physical memory as kmem without running short
 >          * of kmem virtual space.
 >          */
 >         npages = (physmem * 10) / 9;
 > #else
 > 
 > #if defined(KMSAN)
 >         npages = (physmem / 4);
 > #elif defined(PMAP_MAP_POOLPAGE)
 >         npages = (physmem / 4);
 > #else
 >         npages = (physmem / 3) * 2;
 > #endif /* defined(PMAP_MAP_POOLPAGE) */
 > 
 > #if !defined(NKMEMPAGES_MAX_UNLIMITED)
 >         if (npages > NKMEMPAGES_MAX)
 >                 npages = NKMEMPAGES_MAX;
 > #endif
 > 
 > #endif
 > 
 >         if (npages < NKMEMPAGES_MIN)
 >                 npages = NKMEMPAGES_MIN;
 > 
 >         nkmempages = npages;
 > 
 > defined(NKMEMPAGES_MAX_UNLIMITED) && !defined(KMSAN) is running fine now.
 > 
 > I wonder whether we could hit the starvation scenario where ZFS does not
 > reclaim memory in time in the other cases as there nkmempages is set to
 > a value less than physmem.
 > 
 > Just wondering.
 
 
 all of the other cases (except for KMSAN) only exist in 32-bit kernels,
 and I don't think that it's worthwhile to try to make ZFS work in a 32-bit kernel.
 
 I have not thought about what to do for the combination of KMSAN and ZFS.
 I guess I'm leaving that for a rainy day.
 
 -Chuck