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Matthew Ahrens 3442c2a02d
Revise ARC shrinker algorithm 
The ARC shrinker callback `arc_shrinker_count/_scan()` is invoked by the
kernel's shrinker mechanism when the system is running low on free
pages.  This happens via 2 code paths:

1. "direct reclaim": The system is attempting to allocate a page, but we
are low on memory.  The ARC shrinker callback is invoked from the
page-allocation code path.

2. "indirect reclaim": kswapd notices that there aren't many free pages,
so it invokes the ARC shrinker callback.

In both cases, the kernel's shrinker code requests that the ARC shrinker
callback release some of its cache, and then it measures how many pages
were released.  However, it's measurement of released pages does not
include pages that are freed via `__free_pages()`, which is how the ARC
releases memory (via `abd_free_chunks()`).  Rather, the kernel shrinker
code is looking for pages to be placed on the lists of reclaimable pages
(which is separate from actually-free pages).

Because the kernel shrinker code doesn't detect that the ARC has
released pages, it may call the ARC shrinker callback many times,
resulting in the ARC "collapsing" down to `arc_c_min`.  This has several
negative impacts:

1. ZFS doesn't use RAM to cache data effectively.

2. In the direct reclaim case, a single page allocation may wait a long
time (e.g. more than a minute) while we evict the entire ARC.

3. Even with the improvements made in 67c0f0dedc ("ARC shrinking blocks
reads/writes"), occasionally `arc_size` may stay above `arc_c` for the
entire time of the ARC collapse, thus blocking ZFS read/write operations
in `arc_get_data_impl()`.

To address these issues, this commit limits the ways that the ARC
shrinker callback can be used by the kernel shrinker code, and mitigates
the impact of arc_is_overflowing() on ZFS read/write operations.

With this commit:

1. We limit the amount of data that can be reclaimed from the ARC via
the "direct reclaim" shrinker.  This limits the amount of time it takes
to allocate a single page.

2. We do not allow the ARC to shrink via kswapd (indirect reclaim).
Instead we rely on `arc_evict_zthr` to monitor free memory and reduce
the ARC target size to keep sufficient free memory in the system.  Note
that we can't simply rely on limiting the amount that we reclaim at once
(as for the direct reclaim case), because kswapd's "boosted" logic can
invoke the callback an unlimited number of times (see
`balance_pgdat()`).

3. When `arc_is_overflowing()` and we want to allocate memory,
`arc_get_data_impl()` will wait only for a multiple of the requested
amount of data to be evicted, rather than waiting for the ARC to no
longer be overflowing.  This allows ZFS reads/writes to make progress
even while the ARC is overflowing, while also ensuring that the eviction
thread makes progress towards reducing the total amount of memory used
by the ARC.

4. The amount of memory that the ARC always tries to keep free for the
rest of the system, `arc_sys_free` is increased.

5. Now that the shrinker callback is able to provide feedback to the
kernel's shrinker code about our progress, we can safely enable
the kswapd hook. This will allow the arc to receive notifications
when memory pressure is first detected by the kernel. We also
re-enable the appropriate kstats to track these callbacks.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: George Wilson <george.wilson@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10600
2020-07-31 21:10:52 -07:00
.github OpenZFSify CONTRIBUTING.md 2020-07-22 11:09:04 -07:00
cmd Rename refcount.h to zfs_refcount.h 2020-07-29 16:35:33 -07:00
config Use correct prefix for share/pam-configs 2020-07-30 09:09:46 -07:00
contrib pyzfs: Add missing entry to zfs_errno 2020-07-31 09:01:41 -07:00
etc Verify zfs module loaded before starting services 2020-07-29 16:52:18 -07:00
include Revise ARC shrinker algorithm 2020-07-31 21:10:52 -07:00
lib When encountering EZFS_UNKNOWN, print the error text buffer anyway 2020-07-31 09:07:37 -07:00
man Revise ARC shrinker algorithm 2020-07-31 21:10:52 -07:00
module Revise ARC shrinker algorithm 2020-07-31 21:10:52 -07:00
rpm Use correct prefix for share/pam-configs 2020-07-30 09:09:46 -07:00
scripts Add zfs_gitrev.h to the distributed sources 2020-07-22 10:00:40 -07:00
tests ZTS: zvol_misc_volmode is flaky on FreeBSD 2020-07-31 21:05:55 -07:00
udev Centralize variable substitution 2020-07-14 17:33:44 -07:00
.editorconfig Add an .editorconfig; document git whitespace settings 2020-01-27 13:32:52 -08:00
.gitignore Add FreeBSD support to OpenZFS 2020-04-14 11:36:28 -07:00
.gitmodules Add zimport.sh compatibility test script 2014-02-21 12:10:31 -08:00
.travis.yml Add .travis.yml 2017-11-13 09:18:18 -08:00
AUTHORS Update build system and packaging 2018-05-29 16:00:33 -07:00
autogen.sh Cause autogen.sh to fail if autoreconf fails 2018-07-06 09:27:37 -07:00
CODE_OF_CONDUCT.md Add CODE_OF_CONDUCT.md 2019-04-30 10:58:45 -07:00
configure.ac Clean up lib dependencies 2020-07-10 14:26:00 -07:00
copy-builtin Move zfs_gitrev.h to build directory 2020-06-24 18:19:28 -07:00
COPYRIGHT Fix typos 2020-06-09 21:24:09 -07:00
cppcheck-suppressions.txt Move cppcheck suppressions out of .github 2020-06-27 17:40:15 -07:00
LICENSE Update build system and packaging 2018-05-29 16:00:33 -07:00
Makefile.am Add zfs_gitrev.h to the distributed sources 2020-07-22 10:00:40 -07:00
META Linux 5.7 compat: blk_alloc_queue() 2020-04-09 09:16:46 -07:00
NEWS Add NEWS file 2018-09-18 12:03:47 -07:00
NOTICE Update build system and packaging 2018-05-29 16:00:33 -07:00
README.md Update wiki links with new address 2020-06-03 19:46:31 -07:00
TEST Remove CI builder customization from TEST 2020-03-16 10:46:03 -07:00
zfs.release.in Move zfs.release generation to configure step 2012-07-12 12:22:51 -07:00

README.md

img

OpenZFS is an advanced file system and volume manager which was originally developed for Solaris and is now maintained by the OpenZFS community. This repository contains the code for running OpenZFS on Linux and FreeBSD.

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Official Resources

Installation

Full documentation for installing OpenZFS on your favorite Linux distribution can be found at the ZoL Site.

Contribute & Develop

We have a separate document with contribution guidelines.

We have a Code of Conduct.

Release

OpenZFS is released under a CDDL license. For more details see the NOTICE, LICENSE and COPYRIGHT files; UCRL-CODE-235197

Supported Kernels

  • The META file contains the officially recognized supported Linux kernel versions.
  • Supported FreeBSD versions are 12-STABLE and 13-CURRENT.