[Bug 266302] [zfs][iscsi] Periodic drops by ctl with "failed to allocate soft PDU" since 13.1

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=266302

John Baldwin <jhb@FreeBSD.org> changed:

           What    |Removed                     |Added
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                 CC|                            |markj@FreeBSD.org

--- Comment #4 from John Baldwin <jhb@FreeBSD.org> ---
I think the large number of frees of various items is probably due to the
vm_lowmem handler running due to the system running out of memory, and that the
30s probably is simply too large of a granularity to notice the event.  I think
you'd have to do more work to figure out what is allocating more memory during
the spike and why.  I do think iSCSI is just a victim here as the relevant UMA
zone doesn't show up in any of your graphs as a top memory consumer, just for
allocation failures.  Note that you also had a fairly big spike of vm_pagcache2
at the time of failure, and a spike in requests for several ZFS-related zones. 
The zio_buf_comb_1048576 wasn't in the request graph at all until the spike at
the very end.  I don't know if that's another indication that vm_lowmem was
triggered or not.

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