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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
111 lines
3.4 KiB
C
111 lines
3.4 KiB
C
/*
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* Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
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* Copyright (C) 2007 The Regents of the University of California.
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* Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
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* Written by Brian Behlendorf <behlendorf1@llnl.gov>.
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* UCRL-CODE-235197
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*
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* This file is part of the SPL, Solaris Porting Layer.
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* For details, see <http://zfsonlinux.org/>.
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*
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* The SPL is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License as published by the
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* Free Software Foundation; either version 2 of the License, or (at your
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* option) any later version.
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*
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* The SPL is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with the SPL. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef _SPL_SHRINKER_H
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#define _SPL_SHRINKER_H
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#include <linux/mm.h>
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#include <linux/fs.h>
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/*
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* Due to frequent changes in the shrinker API the following
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* compatibility wrappers should be used. They are as follows:
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*
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* SPL_SHRINKER_DECLARE(varname, countfunc, scanfunc, seek_cost);
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*
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* SPL_SHRINKER_DECLARE is used to declare a shrinker with the name varname,
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* which is passed to spl_register_shrinker()/spl_unregister_shrinker().
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* The countfunc returns the number of free-able objects.
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* The scanfunc returns the number of objects that were freed.
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* The callbacks can return SHRINK_STOP if further calls can't make any more
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* progress. Note that a return value of SHRINK_EMPTY is currently not
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* supported.
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*
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* Example:
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*
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* static unsigned long
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* my_count(struct shrinker *shrink, struct shrink_control *sc)
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* {
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* ...calculate number of objects in the cache...
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*
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* return (number of objects in the cache);
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* }
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*
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* static unsigned long
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* my_scan(struct shrinker *shrink, struct shrink_control *sc)
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* {
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* ...scan objects in the cache and reclaim them...
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* }
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*
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* SPL_SHRINKER_DECLARE(my_shrinker, my_count, my_scan, DEFAULT_SEEKS);
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*
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* void my_init_func(void) {
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* spl_register_shrinker(&my_shrinker);
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* }
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*/
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#define spl_register_shrinker(x) register_shrinker(x)
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#define spl_unregister_shrinker(x) unregister_shrinker(x)
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/*
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* Linux 3.0 to 3.11 Shrinker API Compatibility.
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*/
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#if defined(HAVE_SINGLE_SHRINKER_CALLBACK)
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#define SPL_SHRINKER_DECLARE(varname, countfunc, scanfunc, seek_cost) \
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static int \
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__ ## varname ## _wrapper(struct shrinker *shrink, struct shrink_control *sc)\
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{ \
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if (sc->nr_to_scan != 0) { \
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(void) scanfunc(shrink, sc); \
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} \
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return (countfunc(shrink, sc)); \
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} \
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\
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static struct shrinker varname = { \
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.shrink = __ ## varname ## _wrapper, \
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.seeks = seek_cost, \
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}
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#define SHRINK_STOP (-1)
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/*
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* Linux 3.12 and later Shrinker API Compatibility.
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*/
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#elif defined(HAVE_SPLIT_SHRINKER_CALLBACK)
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#define SPL_SHRINKER_DECLARE(varname, countfunc, scanfunc, seek_cost) \
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static struct shrinker varname = { \
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.count_objects = countfunc, \
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.scan_objects = scanfunc, \
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.seeks = seek_cost, \
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}
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#else
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/*
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* Linux 2.x to 2.6.22, or a newer shrinker API has been introduced.
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*/
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#error "Unknown shrinker callback"
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#endif
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#endif /* SPL_SHRINKER_H */
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