linux/Documentation/slow-work.txt
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   1                     ====================================
   2                     SLOW WORK ITEM EXECUTION THREAD POOL
   3                     ====================================
   4
   5By: David Howells <dhowells@redhat.com>
   6
   7The slow work item execution thread pool is a pool of threads for performing
   8things that take a relatively long time, such as making mkdir calls.
   9Typically, when processing something, these items will spend a lot of time
  10blocking a thread on I/O, thus making that thread unavailable for doing other
  11work.
  12
  13The standard workqueue model is unsuitable for this class of work item as that
  14limits the owner to a single thread or a single thread per CPU.  For some
  15tasks, however, more threads - or fewer - are required.
  16
  17There is just one pool per system.  It contains no threads unless something
  18wants to use it - and that something must register its interest first.  When
  19the pool is active, the number of threads it contains is dynamic, varying
  20between a maximum and minimum setting, depending on the load.
  21
  22
  23====================
  24CLASSES OF WORK ITEM
  25====================
  26
  27This pool support two classes of work items:
  28
  29 (*) Slow work items.
  30
  31 (*) Very slow work items.
  32
  33The former are expected to finish much quicker than the latter.
  34
  35An operation of the very slow class may do a batch combination of several
  36lookups, mkdirs, and a create for instance.
  37
  38An operation of the ordinarily slow class may, for example, write stuff or
  39expand files, provided the time taken to do so isn't too long.
  40
  41Operations of both types may sleep during execution, thus tying up the thread
  42loaned to it.
  43
  44
  45THREAD-TO-CLASS ALLOCATION
  46--------------------------
  47
  48Not all the threads in the pool are available to work on very slow work items.
  49The number will be between one and one fewer than the number of active threads.
  50This is configurable (see the "Pool Configuration" section).
  51
  52All the threads are available to work on ordinarily slow work items, but a
  53percentage of the threads will prefer to work on very slow work items.
  54
  55The configuration ensures that at least one thread will be available to work on
  56very slow work items, and at least one thread will be available that won't work
  57on very slow work items at all.
  58
  59
  60=====================
  61USING SLOW WORK ITEMS
  62=====================
  63
  64Firstly, a module or subsystem wanting to make use of slow work items must
  65register its interest:
  66
  67         int ret = slow_work_register_user();
  68
  69This will return 0 if successful, or a -ve error upon failure.
  70
  71
  72Slow work items may then be set up by:
  73
  74 (1) Declaring a slow_work struct type variable:
  75
  76        #include <linux/slow-work.h>
  77
  78        struct slow_work myitem;
  79
  80 (2) Declaring the operations to be used for this item:
  81
  82        struct slow_work_ops myitem_ops = {
  83                .get_ref = myitem_get_ref,
  84                .put_ref = myitem_put_ref,
  85                .execute = myitem_execute,
  86        };
  87
  88     [*] For a description of the ops, see section "Item Operations".
  89
  90 (3) Initialising the item:
  91
  92        slow_work_init(&myitem, &myitem_ops);
  93
  94     or:
  95
  96        vslow_work_init(&myitem, &myitem_ops);
  97
  98     depending on its class.
  99
 100A suitably set up work item can then be enqueued for processing:
 101
 102        int ret = slow_work_enqueue(&myitem);
 103
 104This will return a -ve error if the thread pool is unable to gain a reference
 105on the item, 0 otherwise.
 106
 107
 108The items are reference counted, so there ought to be no need for a flush
 109operation.  When all a module's slow work items have been processed, and the
 110module has no further interest in the facility, it should unregister its
 111interest:
 112
 113        slow_work_unregister_user();
 114
 115
 116===============
 117ITEM OPERATIONS
 118===============
 119
 120Each work item requires a table of operations of type struct slow_work_ops.
 121All members are required:
 122
 123 (*) Get a reference on an item:
 124
 125        int (*get_ref)(struct slow_work *work);
 126
 127     This allows the thread pool to attempt to pin an item by getting a
 128     reference on it.  This function should return 0 if the reference was
 129     granted, or a -ve error otherwise.  If an error is returned,
 130     slow_work_enqueue() will fail.
 131
 132     The reference is held whilst the item is queued and whilst it is being
 133     executed.  The item may then be requeued with the same reference held, or
 134     the reference will be released.
 135
 136 (*) Release a reference on an item:
 137
 138        void (*put_ref)(struct slow_work *work);
 139
 140     This allows the thread pool to unpin an item by releasing the reference on
 141     it.  The thread pool will not touch the item again once this has been
 142     called.
 143
 144 (*) Execute an item:
 145
 146        void (*execute)(struct slow_work *work);
 147
 148     This should perform the work required of the item.  It may sleep, it may
 149     perform disk I/O and it may wait for locks.
 150
 151
 152==================
 153POOL CONFIGURATION
 154==================
 155
 156The slow-work thread pool has a number of configurables:
 157
 158 (*) /proc/sys/kernel/slow-work/min-threads
 159
 160     The minimum number of threads that should be in the pool whilst it is in
 161     use.  This may be anywhere between 2 and max-threads.
 162
 163 (*) /proc/sys/kernel/slow-work/max-threads
 164
 165     The maximum number of threads that should in the pool.  This may be
 166     anywhere between min-threads and 255 or NR_CPUS * 2, whichever is greater.
 167
 168 (*) /proc/sys/kernel/slow-work/vslow-percentage
 169
 170     The percentage of active threads in the pool that may be used to execute
 171     very slow work items.  This may be between 1 and 99.  The resultant number
 172     is bounded to between 1 and one fewer than the number of active threads.
 173     This ensures there is always at least one thread that can process very
 174     slow work items, and always at least one thread that won't.
 175
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