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** Changed in: linux (Ubuntu)
Status: New => Incomplete
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https://bugs.launchpad.net/bugs/1862822
Title:
init_on_alloc=1 causes big performance regression for ZFS
Status in linux package in Ubuntu:
Incomplete
Bug description:
Apologies if this is not the right forum to raise this concern. If
not please point me to a better place to have this discussion.
The 5.3 linux kernel added a new feature which allows pages to be
zeroed when allocating or freeing them: init_on_alloc and
init_on_free. The init_on_alloc flag is enabled by default on the
Ubuntu 18.04 HWE kernel.
Linux kernel commit:
https://github.com/torvalds/linux/commit/6471384af2a6530696fc0203bafe4de41a23c9ef
ZFS allocates and frees pages frequently (via the ABD structure), e.g.
for every disk access. The additional overhead of zeroing these pages
is significant. For example, I measured a ~40% regression in
performance of an uncached "zfs send ... >/dev/null". While this is a
specific workload, this is a general problem because whenever we read
from disk we are going to allocate pages, unnecessarily zero them, and
then immediately fill them with the data from disk.
I don't see any mechanism for ZFS to request that pages *not* be
zeroed when they are allocated.
This performance regression has been noted by users of ZFS on Ubuntu:
https://github.com/zfsonlinux/zfs/issues/9910
I'd like to request that the default in Ubuntu be changed to
init_on_alloc=0, so that users of ZFS on Ubuntu do not experience this
performance regression.
(FYI, my employer, Delphix, has worked around this by changing the
default in our product, but my concern is that the vast majority of
Ubuntu users will simply experience this as "Ubuntu HWE is much slower
than before".)
$ cat /proc/version_signature
Ubuntu 5.3.0-28.30~18.04.1-generic 5.3.13
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