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

[Frank Kardel <kardel%netbsd.org@localhost>]

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

From: Frank Kardel <kardel%netbsd.org@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/57558: pgdaemon 100% busy - no scanning (ZFS case)
Date: Thu, 2 May 2024 10:49:17 +0200

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 In order to prevent the pagedaemon looping and kva space being exhausted
 ZFS not only needs to ensure that uvmexp.freetarg pages are free but 
 also that
 the at least 10% free kva is obeyed for the sake of the pagedaemon/kva 
 free space.
 
 Thus arc..c needs an additional check to determine needfree if the kva 
 free space falls below
 10%.
 
 This was tested with the db load scenario on a Xen DOMU with GENERIC 
 kernel and
 360GB memory. The system stayed responsive an used around 270-319GB pool 
 space, ZFS cut back on pool memory when needed.
 In another test a runaway memory consumer got the pool space used to cut 
 back
 from 319 GB to 51GB before hitting "out of swap space".
 
 
 
 
 
 
 
 
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 Index: arc.c
 ===================================================================
 RCS file: /cvsroot/src/external/cddl/osnet/dist/uts/common/fs/zfs/arc.c,v
 retrieving revision 1.22
 diff -u -r1.22 arc.c
 --- arc.c	3 Aug 2022 01:53:06 -0000	1.22
 +++ arc.c	2 May 2024 08:38:40 -0000
 @@ -3903,6 +3903,31 @@
  	free_memory_reason_t r = FMR_UNKNOWN;
  
  #ifdef _KERNEL
 +#ifdef __NetBSD__
 +	vmem_size_t totalpercent;
 +	vmem_size_t free;
 +
 +	/*
 +	 * PR kern/57558:
 +	 *
 +	 * do not let pdaemon get stuck in the uvm_km_va_starved_p()
 +	 * state. it starts a tight loop when in uvm_km_va_starved state
 +	 * and ZFS is not freeing any pool pages as it started freeing
 +	 * only when falling below uvmexp.freetarg.
 +	 * now we start freeing when falling below 10% kva free or
 +	 * uvmexp.freetarg.
 +	 * the 10% magic is shamelessly copied from uvm_km_va_starved_p()
 +	 * The interface to the pagedaemon has room for improvement.
 +	 */
 +
 +	totalpercent = vmem_size(heap_arena, VMEM_ALLOC|VMEM_FREE) / 10;
 +	free = vmem_size(heap_arena, VMEM_FREE);
 +
 +	if (free < totalpercent) {
 +		needfree = btop(totalpercent - free);
 +	}
 +#endif
 +
  	if (needfree > 0) {
  		n = PAGESIZE * (-needfree);
  		if (n < lowest) {
 
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