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62aa81a577
Add a new defclsyspri macro which can be used to request the default Linux scheduler priority. Neither the minclsyspri or maxclsyspri map to the default Linux kernel thread priority. This makes it awkward to create taskqs which run with the same priority as the rest of the kernel threads on the system which can lead to performance issues. All SPL callers which previously used minclsyspri or maxclsyspri have been changed to use defclsyspri. The vast majority of callers were part of the test suite which won't have an external impact. The few places where it could impact performance the change was from maxclsyspri to defclsyspri. This makes it more likely the process will be scheduled which may help performance. To facilitate further performance analysis the spl_taskq_thread_priority module option has been added. When disabled (0) all newly created kernel threads will use the default kernel thread priority. When enabled (1) the specified taskq priority will be used. By default this value is enabled (1). Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
301 lines
9.4 KiB
Groff
301 lines
9.4 KiB
Groff
'\" te
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.\"
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.\" Copyright 2013 Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
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.\"
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.TH SPL-MODULE-PARAMETERS 5 "Nov 18, 2013"
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.SH NAME
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spl\-module\-parameters \- SPL module parameters
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.SH DESCRIPTION
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.sp
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.LP
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Description of the different parameters to the SPL module.
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.SS "Module parameters"
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.sp
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.LP
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_expire\fR (uint)
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.ad
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.RS 12n
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Cache expiration is part of default Illumos cache behavior. The idea is
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that objects in magazines which have not been recently accessed should be
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returned to the slabs periodically. This is known as cache aging and
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when enabled objects will be typically returned after 15 seconds.
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.sp
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On the other hand Linux slabs are designed to never move objects back to
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the slabs unless there is memory pressure. This is possible because under
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Linux the cache will be notified when memory is low and objects can be
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released.
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.sp
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By default only the Linux method is enabled. It has been shown to improve
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responsiveness on low memory systems and not negatively impact the performance
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of systems with more memory. This policy may be changed by setting the
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\fBspl_kmem_cache_expire\fR bit mask as follows, both policies may be enabled
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concurrently.
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.sp
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0x01 - Aging (Illumos), 0x02 - Low memory (Linux)
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.sp
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Default value: \fB0x02\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_reclaim\fR (uint)
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.ad
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.RS 12n
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When this is set it prevents Linux from being able to rapidly reclaim all the
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memory held by the kmem caches. This may be useful in circumstances where
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it's preferable that Linux reclaim memory from some other subsystem first.
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Setting this will increase the likelihood out of memory events on a memory
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constrained system.
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.sp
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Default value: \fB0\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_obj_per_slab\fR (uint)
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.ad
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.RS 12n
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The preferred number of objects per slab in the cache. In general, a larger
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value will increase the caches memory footprint while decreasing the time
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required to perform an allocation. Conversely, a smaller value will minimize
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the footprint and improve cache reclaim time but individual allocations may
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take longer.
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.sp
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Default value: \fB8\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_obj_per_slab_min\fR (uint)
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.ad
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.RS 12n
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The minimum number of objects allowed per slab. Normally slabs will contain
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\fBspl_kmem_cache_obj_per_slab\fR objects but for caches that contain very
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large objects it's desirable to only have a few, or even just one, object per
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slab.
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.sp
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Default value: \fB1\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_max_size\fR (uint)
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.ad
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.RS 12n
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The maximum size of a kmem cache slab in MiB. This effectively limits
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the maximum cache object size to \fBspl_kmem_cache_max_size\fR /
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\fBspl_kmem_cache_obj_per_slab\fR. Caches may not be created with
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object sized larger than this limit.
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.sp
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Default value: \fB32 (64-bit) or 4 (32-bit)\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_slab_limit\fR (uint)
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.ad
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.RS 12n
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For small objects the Linux slab allocator should be used to make the most
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efficient use of the memory. However, large objects are not supported by
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the Linux slab and therefore the SPL implementation is preferred. This
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value is used to determine the cutoff between a small and large object.
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.sp
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Objects of \fBspl_kmem_cache_slab_limit\fR or smaller will be allocated
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using the Linux slab allocator, large objects use the SPL allocator. A
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cutoff of 16K was determined to be optimal for architectures using 4K pages.
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.sp
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Default value: \fB16,384\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_kmem_limit\fR (uint)
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.ad
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.RS 12n
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Depending on the size of a cache object it may be backed by kmalloc()'d
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or vmalloc()'d memory. This is because the size of the required allocation
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greatly impacts the best way to allocate the memory.
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.sp
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When objects are small and only a small number of memory pages need to be
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allocated, ideally just one, then kmalloc() is very efficient. However,
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when allocating multiple pages with kmalloc() it gets increasingly expensive
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because the pages must be physically contiguous.
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.sp
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For this reason we shift to vmalloc() for slabs of large objects which
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which removes the need for contiguous pages. We cannot use vmalloc() in
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all cases because there is significant locking overhead involved. This
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function takes a single global lock over the entire virtual address range
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which serializes all allocations. Using slightly different allocation
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functions for small and large objects allows us to handle a wide range of
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object sizes.
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.sh
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The \fBspl_kmem_cache_kmem_limit\fR value is used to determine this cutoff
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size. One quarter the PAGE_SIZE is used as the default value because
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\fBspl_kmem_cache_obj_per_slab\fR defaults to 16. This means that at
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most we will need to allocate four contiguous pages.
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.sp
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Default value: \fBPAGE_SIZE/4\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_alloc_warn\fR (uint)
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.ad
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.RS 12n
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As a general rule kmem_alloc() allocations should be small, preferably
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just a few pages since they must by physically contiguous. Therefore, a
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rate limited warning will be printed to the console for any kmem_alloc()
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which exceeds a reasonable threshold.
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.sp
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The default warning threshold is set to eight pages but capped at 32K to
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accommodate systems using large pages. This value was selected to be small
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enough to ensure the largest allocations are quickly noticed and fixed.
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But large enough to avoid logging any warnings when a allocation size is
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larger than optimal but not a serious concern. Since this value is tunable,
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developers are encouraged to set it lower when testing so any new largish
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allocations are quickly caught. These warnings may be disabled by setting
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the threshold to zero.
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.sp
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Default value: \fB32,768\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_alloc_max\fR (uint)
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.ad
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.RS 12n
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Large kmem_alloc() allocations will fail if they exceed KMALLOC_MAX_SIZE.
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Allocations which are marginally smaller than this limit may succeed but
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should still be avoided due to the expense of locating a contiguous range
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of free pages. Therefore, a maximum kmem size with reasonable safely
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margin of 4x is set. Kmem_alloc() allocations larger than this maximum
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will quickly fail. Vmem_alloc() allocations less than or equal to this
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value will use kmalloc(), but shift to vmalloc() when exceeding this value.
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.sp
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Default value: \fBKMALLOC_MAX_SIZE/4\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_kmem_cache_magazine_size\fR (uint)
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.ad
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.RS 12n
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Cache magazines are an optimization designed to minimize the cost of
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allocating memory. They do this by keeping a per-cpu cache of recently
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freed objects, which can then be reallocated without taking a lock. This
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can improve performance on highly contended caches. However, because
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objects in magazines will prevent otherwise empty slabs from being
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immediately released this may not be ideal for low memory machines.
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.sp
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For this reason \fBspl_kmem_cache_magazine_size\fR can be used to set a
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maximum magazine size. When this value is set to 0 the magazine size will
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be automatically determined based on the object size. Otherwise magazines
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will be limited to 2-256 objects per magazine (i.e per cpu). Magazines
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may never be entirely disabled in this implementation.
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.sp
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Default value: \fB0\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_hostid\fR (ulong)
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.ad
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.RS 12n
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The system hostid, when set this can be used to uniquely identify a system.
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By default this value is set to zero which indicates the hostid is disabled.
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It can be explicitly enabled by placing a unique non-zero value in
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\fB/etc/hostid/\fR.
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.sp
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Default value: \fB0\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_hostid_path\fR (charp)
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.ad
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.RS 12n
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The expected path to locate the system hostid when specified. This value
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may be overridden for non-standard configurations.
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.sp
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Default value: \fB/etc/hostid\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_taskq_thread_bind\fR (int)
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.ad
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.RS 12n
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Bind taskq threads to specific CPUs. When enabled all taskq threads will
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be distributed evenly over the available CPUs. By default, this behavior
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is disabled to allow the Linux scheduler the maximum flexibility to determine
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where a thread should run.
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.sp
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Default value: \fB0\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_taskq_thread_dynamic\fR (int)
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.ad
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.RS 12n
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Allow dynamic taskqs. When enabled taskqs which set the TASKQ_DYNAMIC flag
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will by default create only a single thread. New threads will be created on
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demand up to a maximum allowed number to facilitate the completion of
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outstanding tasks. Threads which are no longer needed will be promptly
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destroyed. By default this behavior is enabled but it can be disabled to
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aid performance analysis or troubleshooting.
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.sp
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Default value: \fB1\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_taskq_thread_priority\fR (int)
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.ad
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.RS 12n
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Allow newly created taskq threads to set a non-default scheduler priority.
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When enabled the priority specified when a taskq is created will be applied
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to all threads created by that taskq. When disabled all threads will use
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the default Linux kernel thread priority. By default, this behavior is
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enabled.
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.sp
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Default value: \fB1\fR
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.RE
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.sp
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.ne 2
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.na
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\fBspl_taskq_thread_sequential\fR (int)
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.ad
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.RS 12n
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The number of items a taskq worker thread must handle without interruption
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before requesting a new worker thread be spawned. This is used to control
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how quickly taskqs ramp up the number of threads processing the queue.
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Because Linux thread creation and destruction are relatively inexpensive a
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small default value has been selected. This means that normally threads will
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be created aggressively which is desirable. Increasing this value will
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result in a slower thread creation rate which may be preferable for some
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configurations.
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.sp
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Default value: \fB4\fR
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.RE
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