Slow reboots due to ZFS cleanup in kern_shutdown() .. zio_fini()

[Peter Eriksson]

Yep, it’s pmap_remove() that occasionally takes ~1s instead of being very quick. A test I ran:


kmem_unback: pmap_remove(kernel_pmap, 18446741878385815552, 18446741878385827840) took 1 seconds
kmem_free: kmem_unback(kmem_object, 18446741878385815552, 12288) took 1 seconds
page_free: kmem_free(kmem_arena, 18446741878385815552, 12288) took 1 seconds
keg_free_slab: keg->uk_freef(mem) {page_free} took 1 seconds

keg_drain: while()-keg_free_slab-loop took 17 seconds [23136 loops, 17 slow calls]
zone_drain_wait(): zone_foreach_keg(zone, &keg_drain) took 17 seconds
zone_dtor(): zone_drain_wait(zone, M_WAITOK) took 17 seconds
zone_free_item(zone=UMA Zones): zone->uz_dtor() took 17 seconds
uma_zdestroy(zio_buf_12288) took 17 seconds
kmem_cache_destroy: uma_zdestroy(0xfffff803465dd980) [zio_buf_12288] took 17 seconds
kmem_cache_destroy(zio_buf_cache[20]) took 17 seconds


Ie, 17 out of 23136 calls to keg_free_slab()->page_free()->kmem_free()->kmem_unback()->pmap_remove() was slow (took atleast 1s).

I created a bug report (242427) on bugs.freebsd.org <http://bugs.freebsd.org/> for this now in case someone knows how to debug the PMAP stuff… :-)

- Peter