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

[mlelstv%serpens.de@localhost (Michael van Elst)]

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

From: mlelstv%serpens.de@localhost (Michael van Elst)
To: gnats-bugs%netbsd.org@localhost
Cc: 
Subject: Re: kern/57558: pgdaemon 100% busy - no scanning (ZFS case)
Date: Sat, 5 Aug 2023 13:24:55 -0000 (UTC)

 chuq%chuq.com@localhost (Chuck Silvers) writes:
 
 > this patch is not correct.  it does not do the right thing when there
 > is plenty of KVA but a shortage of physical pages.
 
 I doubt that this is handled well enough outside of zfs.
 
 The result of using ZFS (removing pkgsrc tree, unpacking it again) is
 about 3.2GB of kernel pools used (from 8GB total RAM).
 
 Trying to reduce ZFS again by tuning ZFS, shrinking maxvnodes and
 then allocating user pages ends in the following (only pools >10G shown):
 
 anonpl               0.078G
 arc_buf_hdr_t_f      0.018G
 buf16k               0.014G
 dmu_buf_impl_t       0.107G
 dnode_t              0.173G
 kmem-00064           0.037G
 kmem-00128           0.089G
 kmem-00192           0.182G
 kmem-00256           0.069G
 kmem-00384           0.161G
 kmem-01024           0.020G
 kmem-02048           0.033G
 mutex                0.020G
 namecache            0.020G
 pcglarge             0.042G
 pcgnormal            0.107G
 phpool-64            0.014G
 rwlock               0.020G
 sa_cache             0.032G
 vcachepl             0.206G
 zfs_znode_cache      0.072G
 zio_buf_131072       0.015G
 zio_buf_16384        0.075G
 zio_buf_4096         0.019G
 zio_buf_512          0.170G
 zio_cache            0.161G
 
 That's about 2GB left. Things like dnodes or the zio_cache are never
 flushed, the 512 byte zio buffer pool is still huge because it is
 totally fragmented, but also vcachepl is never drained.
 
 ZFS tries to drop referenced metadata in Solaris or FreeBSD, but
 for NetBSD that's still a nop.
 
 The OpenZFS code in current FreeBSD looks quite different too in
 that area.