sys/getiopolicy_np.3

   1 .Dd July 18, 2006
   2 .Dt getiopolicy_np 3
   3 .Os
   4 .Sh NAME
   5 .Nm getiopolicy_np, setiopolicy_np
   6 .Nd manipulate the I/O policy of a process or thread
   7 .Sh LIBRARY
   8 .Lb libc
   9 .Sh SYNOPSIS
  10 .In sys/resource.h
  11 .Ft int
  12 .Fn getiopolicy_np "int iotype" "int scope"
  13 .Ft int
  14 .Fn setiopolicy_np "int iotype" "int scope" "int policy"
  15 .Sh DESCRIPTION
  16 The
  17 .Fn getiopolicy_np
  18 and
  19 .Fn setiopolicy_np
  20 functions are provided to get or set the I/O policy of the current process
  21 or the current thread.  The policy of the I/O of the given type
  22 .Fa iotype
  23 can be get or set for the given
  24 .Fa scope .
  25 .Pp
  26 The I/O type is specified in the argument
  27 .Fa iotype .
  28 The currently supported I/O type is
  29 .Dv IOPOL_TYPE_DISK ,
  30 which means the I/O policy for I/Os to local disks can be get or set.  I/Os to
  31 local disks are I/Os sent to the media without going through a network,
  32 including I/Os to internal and external hard drives, optical media in internal
  33 and external drives, flash drives, floppy disks, ram disks, and mounted disk
  34 images which reside on these media, but not including remote volumes mounted
  35 through networks (AFP, SMB, NFS, etc) or disk images residing on remote volumes.
  36 .Pp
  37 The scope that the I/O policy takes effect is specified in the argument
  38 .Fa scope
  39 as follows:
  40 .Bl -tag -width IOPOL_SCOPE_PROCESS
  41 .It IOPOL_SCOPE_PROCESS
  42 The I/O policy of all I/Os issued by the current process is get or set.
  43 .It IOPOL_SCOPE_THREAD
  44 The I/O policy of all I/Os issued by the current thread is get or set.
  45 .El
  46 .Pp
  47 In
  48 .Fn getiopolicy_np ,
  49 the I/O policy of the given I/O type and scope is returned.  In
  50 .Fn setiopolicy_np ,
  51 the argument
  52 .Fa policy
  53 is an integer which contains the new I/O policy to be set for the given I/O
  54 type and scope.  The I/O policy can have the following values:
  55 .Bl -tag -width IOPOL_PASSIVEXX
  56 .It IOPOL_DEFAULT
  57 This is the default I/O policy for the first process and every new created thread.
  58 .It IOPOL_NORMAL
  59 I/Os with NORMAL policy are called NORMAL I/Os.  They are handled by the
  60 system using best-effort.
  61 .It IOPOL_THROTTLE
  62 I/Os with THROTTLE policy are called THROTTLE I/Os.  If a THROTTLE I/O request
  63 occurs within a small time window (usually a fraction of a second) of another
  64 NORMAL I/O request, the thread that issues the THROTTLE I/O is forced to sleep
  65 for a certain interval. This slows down the thread that issues the THROTTLE I/O
  66 so that NORMAL I/Os can utilize most of the disk I/O bandwidth.
  67 .It IOPOL_PASSIVE
  68 The PASSIVE I/Os are a special type of NORMAL I/O that are processed the same as
  69 NORMAL I/Os but are ignored by the THROTTLE I/Os so that the threads issuing
  70 THROTTLE I/Os are not slowed down by PASSIVE I/Os.  The PASSIVE I/O policy is
  71 useful for server type applications.  The I/Os generated by these applications
  72 are called passive I/Os because these I/Os are caused directly or indirectly by
  73 the I/O requests they receive from client applications.  For example, when an
  74 image file is mounted by DiskImages, DiskImages generate passive I/Os.
  75 DiskImages should mark these I/Os using the PASSIVE I/O policy so that when
  76 client applications that issue THROTTLE I/Os access the volume managed by
  77 DiskImages, these client applications will not be slowed down by the I/Os
  78 generated by DiskImages.
  79 .El
  80 .Pp
  81 The I/O policy of a new created process is inherited from its parent
  82 process.  The I/O policy of an I/O request depends on the I/O policy of
  83 both the current thread and the current process.  If the I/O policy of the
  84 current thread is IOPOL_DEFAULT, the I/O policy of the current process is
  85 used; if the I/O policy of the current thread is not IOPOL_DEFAULT, the
  86 I/O policy of the current thread overrides the I/O policy of the current
  87 process; if the I/O policy of the current process is IOPOL_DEFAULT, the
  88 policy of I/Os issued by this process is NORMAL.  For example, given the
  89 following thread and process I/O policy in the first two columns, the I/O
  90 policy of all I/Os issued by the thread is given in the third column:
  91 .Bl -column "Process I/O ScopeXXX" "Thread I/O ScopeXXX" "I/O Policy" -offset indent
  92 .It Sy "Process I/O Policy      Thread I/O Policy       I/O Policy"
  93 .It "DEFAULT    DEFAULT NORMAL"
  94 .It "DEFAULT    PASSIVE PASSIVE"
  95 .It "THROTTLE   DEFAULT THROTTLE"
  96 .It "THROTTLE   PASSIVE PASSIVE"
  97 .It "PASSIVE    NORMAL  NORMAL"
  98 .El
  99 .Pp
 100 The thread or process with THROTTLE I/O policy enabled may be slowed down when
 101 it issues reads, but will not be slowed down when it issues writes.
 102 If it issues far more writes than reads (e.g., an application
 103 downloading large amounts of data through the network), these writes compete with the
 104 normal I/Os of other processes and may have an adverse effect on the I/O
 105 throughput or latency of those processes.
 106 .Pp
 107 .Sh RETURN VALUES
 108 The
 109 .Fn getiopolicy_np
 110 call returns the I/O policy of the given I/O type and scope.  If error
 111 happens, -1 is returned.  The
 112 .Fn setiopolicy_np
 113 call returns 0 if there is no error, or -1 if there is an error.  When error
 114 happens, the error code is stored in the external variable
 115 .Fa errno .
 116 .Sh ERRORS
 117 .Fn Getiopolicy_np
 118 and
 119 .Fn setiopolicy_np
 120 will fail if:
 121 .Bl -tag -width Er
 122 .It Bq Er EINVAL
 123 Io_type or scope is not one of the values defined in this manual.
 124 .El
 125 .Pp
 126 In addition to the errors indicated above,
 127 .Fn setiopolicy_np
 128 will fail if:
 129 .Bl -tag -width Er
 130 .It Bq Er EINVAL
 131 Policy is not one of the values defined in this manual.
 132 .El
 133 .Sh SEE ALSO
 134 .Xr nice 3 ,
 135 .Xr getpriority 2 ,
 136 .Xr setpriority 2 ,
 137 .Xr renice 8
 138 .Sh HISTORY
 139 The
 140 .Fn getiopolicy_np
 141 and
 142 .Fn setiopolicy_np
 143 function call first appeared in Mac OS X 10.5 (Leopard) .