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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
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| .. | ||
| abd_os.c | ||
| arc_os.c | ||
| crypto_os.c | ||
| dmu_os.c | ||
| hkdf.c | ||
| kmod_core.c | ||
| spa_os.c | ||
| spa_stats.c | ||
| sysctl_os.c | ||
| vdev_file.c | ||
| vdev_geom.c | ||
| vdev_label_os.c | ||
| zfs_acl.c | ||
| zfs_ctldir.c | ||
| zfs_debug.c | ||
| zfs_dir.c | ||
| zfs_file_os.c | ||
| zfs_fuid_os.c | ||
| zfs_ioctl_compat.c | ||
| zfs_ioctl_os.c | ||
| zfs_onexit_os.c | ||
| zfs_vfsops.c | ||
| zfs_vnops.c | ||
| zfs_znode.c | ||
| zio_crypt.c | ||
| zvol_os.c | ||