Re: port-evbarm/56944: ZFS heavy usage on NetBSD running in Mac M1 VM results in kernel thread running away and filesystem hang

[Tobias Nygren <tnn%NetBSD.org@localhost>]

The following reply was made to PR port-evbarm/56944; it has been noted by GNATS.

From: Tobias Nygren <tnn%NetBSD.org@localhost>
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: port-evbarm/56944: ZFS heavy usage on NetBSD running in Mac M1
 VM results in kernel thread running away and filesystem hang
Date: Wed, 27 Jul 2022 19:18:27 +0200

 When pagedaemon is spinning it is indicative of a memory pressure
 situation that is unresolvable. The interaction between pagedaemon and
 zfs is primarily ARC reclamation. Some observations:
 
 1) It doesn't look like we initialise zfs_arc_free_target, unlike FreeBSD.
 2) FreeBSD has additional code to check for kva fragmentation which
    we do not.
 
 So it might be worthwhile to experiment with zfs_arc_free_target to
 preemptively avoid the situation where the kernel fails to reclaim enough
 pages to continue working. Here's a patch for zfs.kmod you could try:
 
 --- external/cddl/osnet/dist/uts/common/fs/zfs/arc.c	4 May 2022 15:49:55 -0000	1.21
 +++ external/cddl/osnet/dist/uts/common/fs/zfs/arc.c	27 Jul 2022 17:10:16 -0000
 @@ -387,7 +387,7 @@ int zfs_arc_grow_retry = 0;
  int zfs_arc_shrink_shift = 0;
  int zfs_arc_p_min_shift = 0;
  uint64_t zfs_arc_average_blocksize = 8 * 1024; /* 8KB */
 -u_int zfs_arc_free_target = 0;
 +u_int zfs_arc_free_target = 32 * 1024 * 1024;
 
  /* Absolute min for arc min / max is 16MB. */
  static uint64_t arc_abs_min = 16 << 20;
 @@ -3919,6 +3919,14 @@ arc_available_memory(void)
  		r = FMR_LOTSFREE;
  	}
 
 +#ifdef __NetBSD__
 +	n = PAGESIZE * ((int64_t)freemem - desfree);
 +	if (n < lowest) {
 +		lowest = n;
 +		r = FMR_LOTSFREE;
 +	}
 +#endif
 +