Another big advantage of memory mapped IO is that it allows you to load a potentially larger file which is not otherwise accessible.
Memory Mapped Files are way faster than standard file access via normal IO. Possibly the main advantage of Memory Mapped IO is the performance which is important to build high-frequency electronic trading system. Advantage and Disadvantage of Memory Mapped File Java programming language supports memory mapped file with the java.nio package and has MappedByteBuffer to read and write from memory. Memory used to load Memory mapped file is outside of Java heap Space. Memory mapped files are special, which allows Java program to access contents directly from memory, this is achieved by mapping whole file or portion of file into memory and operating system takes care of loading page requested and writing into file while application only deals with memory which results in very fast IO operations. No copying/buffering overhead.What is Memory Mapped File and IO in Java The key performance boost comes from the combination with mvec, which can then use data on file as efficiently as in memory. That is why the output below consists of two sections - one with C-level support, and one in pure Tcl.ĭon't put too much emphasis on speed differences as shown below. One of the nice things about "mmap" is that it can be faked entirely in pure Tcl (by reading the entire file on first access).
#MEMORY MAPPED FILE CODE#
The code will be moved into a context and namespace of its own when the dust has settled a bit more.īelow are some examples. This demo includes the C code for mmap (and a few more pieces), with a binary build (just Linux for now). JOLT stands for JC's Own Little Toolbox - it's a context I use to try out new things, especially when mixing Tcl and C and exploring possibilities. When compiled as C extension, "mmap" is part of JOLT. This is the "vkit" project as described elsewhere and is work in progress. The combination mvec+mmap makes it possible to implement high-performance vector storage. The "mmap" command is particular in the sense that "mvec" is special-cased to work with it if present (see "mvec.README"). Even end-of-line translations and end-of-file markers are totally ignored. Keep in mind that mmap is for binary data - it's 100% ignorant of Unicode. A simple way to do this, is to store the channel identifier in a variable, and never use that variable in any other way than as first arg to "mmap". That means that you need to be careful to rarely lose the dual-object representation. Accessing data in this way is extremely efficient, but setting up / tearing down such a mapping is not that cheap. Trivial, eh? You can throw away the "read" command now :)Īctually, things are slightly more complex. The result is a 17-byte string (byte array, to be precise), as found at offset 123 in the file. The second call is the one to fetch a particular byte range: set offset 123 This returns the file size (and sets up the mmap as dual representation). Sockets cannot be used, the file descriptor must be mappable (which is whatever the OS considers acceptable).Īfter that, you only need to deal with two call variations to "mmap": set filesize What you need is an open file in the form of a Tcl "channel" descriptor. The interface is almost hidden, it's very easy to forget it exists (it's also easy to cause "shimmering" and lose most of the speed advantage). Using data in such a way takes maximum advantage of virtual memory (VM). This demo illustrates an experimental "mmap" command and datatype for Tcl, which provides efficient access to files by mapping them into memory.
The "mmap" command for memory mapping is now part of the mvector package in CritLib.
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