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(* $Id: xdr_mstring.mli 1558 2011-03-04 17:15:46Z gerd $ *)
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(** Managed strings are used in XDR context for constant strings that
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are stored either as string or as memory (bigarray of char).
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A managed string [ms] is declared in the XDR file as in
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typedef _managed string ms<>;
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In the encoded XDR stream there is no difference between strings and
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managed strings, i.e. the wire representation is identical. Only
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the Ocaml type differs to which the managed string is mapped. This
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type is {!Xdr_mstring.mstring} (below).
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In the RPC context there is often the problem that the I/O backend
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would profit from a different string representation than the user of
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the RPC layer. To bridge this gap, managed strings have been invented.
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Generally, the user can determine how to represent strings (usually
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either as an Ocaml string, or as memory), and the I/O backend
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can request to transform to a different representation when this
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leads to an improvement (i.e. copy operations can be saved).
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Only large managed strings result in a speedup of the program
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{2 How to practically use managed strings}
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There are two cases: The encoding case, and the decoding case.
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In the encoding case the [mstring] object is created by the user
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and passed to the RPC library. This happens when a client prepares
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an argument for calling a remote procedure, or when the server
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sends a response back to the caller. In the decoding case the client
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analyzes the response from an RPC call, or the server looks at the
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arguments of an RPC invocation. The difference here is that in the
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encoding case user code can directly create [mstring] objects by
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calling functions of this module, whereas in the decoding case the
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RPC library creates the [mstring] objects.
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For simplicity, let us only look at this problem from the perspective
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{b Encoding.} Image a client wants to call an RPC, and one of the
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arguments is a managed string. This means we finally need an [mstring]
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object that can be put into the argument list of the call.
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This library supports two string representation specially: The normal
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Ocaml [string] type, and {!Netsys_mem.memory} which is actually just
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a bigarray of char's. There are two factories [fac],
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- {!Xdr_mstring.string_based_mstrings}, and
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- {!Xdr_mstring.memory_based_mstrings},
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and both can be used to create the [mstring] to pass to the
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RPC layer. It should be noted that this layer can process the
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[memory] representation a bit better. So, if the original [data]
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value is a string, the factory for [string] should be used, and
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if it is a char bigarray, the factory for [memory] should be used.
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Now, the [mstring] object is created by
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- [let mstring = fac # create_from_string data pos len copy_flag], or by
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- [let mstring = fac # create_from_memory data pos len copy_flag].
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Of course, if [fac] is the factory for strings, the [create_from_string]
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method works better, and if [fac] is for [memory], the [create_from_memory]
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method works better. [pos] and [len] can select a substring of [data].
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If [copy_flag] is [false], the [mstring] object does not copy the data
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if possible, but just keeps a reference to [data] until it is accessed;
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otherwise if [copy_flag] is [true], a copy is made immediately.
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Of couse, delaying the copy is better, but this requires that [data]
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is not modified until the RPC call is completed.
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{b Decoding.} Now, the call is done, and the client looks at the
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result. There is also an [mstring] object in the result. As noted
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above, this [mstring] object was already created by the RPC library
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(and currently this library prefers string-based objects if not
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told otherwise). The user code can now access this [mstring]
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object with the access methods of the [mstring] class (see below).
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As these methods are quite limited, it makes normally only sense
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to output the [mstring] contents to a file descriptor.
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The user can request a different factory for managed strings. The
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function {!Rpc_client.set_mstring_factories} can be used for this
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purpose. (Similar ways exist for managed clients, and for RPC servers.)
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{b Potential.} Before introducing managed strings, a clean analysis
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was done how many copy operations can be avoided by using this
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technique. Example: The first N bytes of a file are taken as
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argument of an RPC call. Instead of reading these bytes into a
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normal Ocaml string, an optimal implementation uses now a [memory]
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buffer for this purpose. This gives:
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- Old implementation with strings and ocamlnet-2:
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Data is copied {b six} times from reading it from the file until
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writing it to the socket.
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- New implementation with memory-based mstrings:
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Data is copied only {b twice}! The first copy reads it from the
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file into the input buffer (a [memory] value), and the second copy
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writes the data into the socket.
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Part of the optimization is that [Unix.read] and [Unix.write]
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do a completely avoidable copy of the data which is prevented by
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switching to {!Netsys_mem.mem_read} and {!Netsys_mem.mem_write},
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respectively. The latter two functions exploit an optimization
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that is only possible when the data is [memory]-typed.
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The possible optimizations for the decoding side of the problem
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are slightly less impressive, but still worth doing it.
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(** The object holding the string value *)
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(** The length of the managed string *)
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method blit_to_string : int -> string -> int -> int -> unit
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(** [blit_to_string mpos s spos len]: Copies the substring of the
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managed string from [mpos] to [mpos+len-1] to the substring of
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[s] from [spos] to [spos+len-1]
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method blit_to_memory : int -> memory -> int -> int -> unit
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(** [blit_to_string mpos mem mempos len]: Copies the substring of the
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managed string from [mpos] to [mpos+len-1] to the substring of
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[mem] from [mempos] to [mempos+len-1]
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method as_string : string * int
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(** Returns the contents as string. It is undefined whether the returned
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string is a copy or the underlying buffer. The int is the position
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where the contents start
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method as_memory : memory * int
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(** Returns the contents as memory. It is undefined whether the returned
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memory is a copy or the underlying buffer. The int is the position
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where the contents start
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method preferred : [ `Memory | `String ]
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(** Whether [as_memory] or [as_string] is cheaper *)
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(** The object creating new [mstring] objects *)
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class type mstring_factory =
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method create_from_string : string -> int -> int -> bool -> mstring
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(** [create_from_string s pos len must_copy]: Creates the [mstring] from the
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sub string of s starting at [pos] with length [len]
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If [must_copy] the mstring object must create a copy. Otherwise
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it can just keep the string passed in.
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method create_from_memory : memory -> int -> int -> bool -> mstring
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(** [create_from_memory m pos len must_copy]: Creates the [mstring] from the
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sub string of m starting at [pos] with length [len]
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If [must_copy] the mstring object must create a copy. Otherwise
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it can just keep the memory passed in.
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val string_based_mstrings : mstring_factory
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(** Uses strings to represent mstrings *)
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val string_to_mstring : ?pos:int -> ?len:int -> string -> mstring
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(** Represent a string as mstring (no copy) *)
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val memory_based_mstrings : mstring_factory
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(** Uses memory to represent mstrings. The memory bigarrays are allocated
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with [Bigarray.Array1.create]
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val memory_to_mstring : ?pos:int -> ?len:int -> memory -> mstring
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(** Represent memory as mstring (no copy) *)
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val paligned_memory_based_mstrings : mstring_factory
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(** Uses memory to represent mstrings. The memory bigarrays are allocated
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with {!Netsys_mem.alloc_memory_pages} if available, and
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[Bigarray.Array1.create] if not.
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val memory_pool_based_mstrings : Netsys_mem.memory_pool -> mstring_factory
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(** Uses memory to represent mstrings. The memory bigarrays are obtained
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from the pool. The length of these mstrings is limited by the
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blocksize of the pool.
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val length_mstrings : mstring list -> int
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(** returns the sum of the lengths of the mstrings *)
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val concat_mstrings : mstring list -> string
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(** concatenates the mstrings and return them as single string. The returned
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string may be shared with one of the mstrings passed in.
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val prefix_mstrings : mstring list -> int -> string
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(** [prefix_mstrings l n]: returns the first [n] chars of the
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concatenated mstrings [l] as single string
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val blit_mstrings_to_memory : mstring list -> memory -> unit
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(** blits the mstrings one after the other to the memory, so that
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they appear there concatenated
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val shared_sub_mstring : mstring -> int -> int -> mstring
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(** [shared_sub_mstring ms pos len]: returns an mstring that includes
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a substring of [ms], starting at [pos], and with [len] bytes.
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The returned mstring shares the buffer with the original mstring [ms]
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val shared_sub_mstrings : mstring list -> int -> int -> mstring list
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(** Same for a list of mstrings *)
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val copy_mstring : mstring -> mstring
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val copy_mstrings : mstring list -> mstring list
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type named_mstring_factories =
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(string, mstring_factory) Hashtbl.t
added
removed
src/netstring/xdr_mstring.mli
