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: Thu, 28 Jul 2022 02:06:50 +0200

 I could easily reproduce the bug on a Raspberry Pi 4 with a bolted-on
 ahcisata adapter. With 3090M free memory it does not seem general
 memory shortage is the reason pgdaemon keeps getting woken. So you can
 disregard the previous patch. It looks rather that the reason is kva
 fragmentation, leak or mismanagement.
 
 One possible problem here is ARC tries to maintain 1/16th of the kernel
 heap free:
 
 https://nxr.netbsd.org/xref/src/external/cddl/osnet/dist/uts/common/fs/zfs/arc.c#3990
 
 But UVM tries to maintain 1/10th free:
 
 https://nxr.netbsd.org/xref/src/sys/uvm/uvm_km.c#902
 
 This means UVM will notice and start to rectify kva shortage before ARC,
 when it really needs to be the other way around.