zfs(openzfs): cap the default ARC at half of RAM for app coexistence - #1487
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zfs(openzfs): cap the default ARC at half of RAM for app coexistence#1487gburd wants to merge 1 commit into
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On OSv the ARC and the application share one physical address space. The existing arc_default_max() for a guest with >= 1 GiB of RAM returns MAX(allmem*5/8, allmem-1GiB), which on a large guest is nearly all of RAM (for example ~31 GiB of a 32 GiB guest). A memory-hungry application such as PostgreSQL, with several GiB of shared_buffers plus many concurrent backends, then has almost no headroom, and a burst (a large checkpoint stacking dirty ARC buffers on top of the backends working set) drives the guest out of memory and aborts it. Since OSv ZFS_MODULE_PARAM is a no-op (zfs_arc_max cannot be set at boot), cap the computed default at half of RAM for guests with more than 2 GiB, so the application always keeps at least half the guest memory. Small guests are unchanged (their fractions already leave room). A future zfs_arc_max wired to the boot cmdline could override this. Follow-on to the OpenZFS keystone (cloudius-systems#1423) and the runtime data pool (cloudius-systems#1478). Signed-off-by: Greg Burd <greg@burd.me>
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Draft / follow-on to #1423 and #1478.
On OSv the ARC and the application share one physical address space.
arc_default_max()for a guest with >= 1 GiB of RAM returnsMAX(allmem*5/8, allmem-1GiB), which on a large guest is nearly all of RAM (about 31 GiB of a 32 GiB guest). A memory-hungry application such as PostgreSQL (multi-GiB shared_buffers plus many concurrent backends) then has almost no headroom, and a checkpoint burst stacking dirty ARC buffers on top of the backends' working set drives the guest OOM and aborts it. This showed up as an OOM at high concurrency on a 32 GiB guest.Since
ZFS_MODULE_PARAMis a no-op on OSv (zfs_arc_maxcannot be set at boot), this caps the computed default at half of RAM for guests with more than 2 GiB, so the application always keeps at least half the guest's memory. Small guests are unchanged.Draft because the proper long-term fix is to wire
zfs_arc_maxto the boot cmdline so this becomes an overridable default rather than a hard cap; kept as a draft PR so the tuning is not lost while the maintainer reviews the ZFS follow-on stack. Edits the trackedmodules/open_zfs/osv/module/os/osv/zfs/arc_os.cdirectly (the OSv platform layer is now source files on master, not a numbered patch).