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

[Chuck Silvers <chuq%chuq.com@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: Sun, 5 May 2024 23:05:35 -0700

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 On Sun, May 05, 2024 at 08:33:03AM +0200, Frank Kardel wrote:
 > The issue is not having enough KVA configured  The issue is that with
 > large (not XEN specific) memory systems the page daemon attempts to keep
 > always at least 10% KVA free. See uvm_km.c:uvm_km_va_starved_p(void) and
 > uvm_pdaemon.c:uvm_pageout(void *arg).
 
 ah yes, that is the problem.  this is really a mismatch between how much kmem space
 is allocated vs. how much kmem space is allowed to be used before the pagedaemon
 tries to reclaim some kmem space.  ZFS is just a victim in this because it happens
 to use a lot of kmem space.
 
 I think the right fix for this is to increase the amount of kmem space that we
 allocate such that all of physical memory can be allocated as kmem without
 the pagedaemon considering the system to be starved for kmem virtual space.
 this means allocating an enough kmem space for 10/9 of physical memory,
 so that even though only 9/10 of kmem virtual space can be used, there is still
 enough kmem virtual space available for all of physical memory.
 
 please try the attached patch.
 
 -Chuck
 
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 Index: src/sys/uvm/uvm_km.c
 ===================================================================
 RCS file: /home/chs/netbsd/cvs/src/sys/uvm/uvm_km.c,v
 retrieving revision 1.165
 diff -u -p -r1.165 uvm_km.c
 --- src/sys/uvm/uvm_km.c	9 Apr 2023 09:00:56 -0000	1.165
 +++ src/sys/uvm/uvm_km.c	5 May 2024 17:02:49 -0000
 @@ -227,7 +227,14 @@ kmeminit_nkmempages(void)
  	}
  
  #if defined(NKMEMPAGES_MAX_UNLIMITED) && !defined(KMSAN)
 -	npages = physmem;
 +	/*
 +	 * 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)
 
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