diff options
Diffstat (limited to 'lib/kernel/doc')
| -rw-r--r-- | lib/kernel/doc/src/file.xml | 220 |
1 files changed, 82 insertions, 138 deletions
diff --git a/lib/kernel/doc/src/file.xml b/lib/kernel/doc/src/file.xml index 2ab35b9b05..3e3c5b3bf3 100644 --- a/lib/kernel/doc/src/file.xml +++ b/lib/kernel/doc/src/file.xml @@ -33,11 +33,14 @@ <description> <p>This module provides an interface to the file system.</p> - <p>On operating systems with thread support, - file operations can be performed in threads of their own, allowing - other Erlang processes to continue executing in parallel with - the file operations. See command-line flag - <c>+A</c> in <seealso marker="erts:erl"><c>erl(1)</c></seealso>.</p> + <warning> + <p>File operations are only guaranteed to appear atomic when going + through the same file server. A NIF or other OS process may observe + intermediate steps on certain operations on some operating systems, + eg. renaming an existing file on Windows, or + <seealso marker="#write_file_info/2"><c>write_file_info/2</c> + </seealso> on any OS at the time of writing.</p> + </warning> <p>Regarding filename encoding, the Erlang VM can operate in two modes. The current mode can be queried using function @@ -1438,8 +1441,12 @@ f.txt: {person, "kalle", 25}. which is 1970-01-01 00:00 UTC.</p></item> </taglist> <p>Default is <c>{time, local}</c>.</p> - <p>If the option <c>raw</c> is set, the file server is not called - and only information about local files is returned.</p> + <p>If the option <c>raw</c> is set, the file server is not called and + only information about local files is returned. Note that this will + break this module's atomicity guarantees as it can race with a + concurrent call to + <seealso marker="#write_file_info/2"><c>write_file_info/1,2</c> + </seealso></p> <note> <p>As file times are stored in POSIX time on most OS, it is faster to query file information with option <c>posix</c>.</p> @@ -1687,8 +1694,12 @@ f.txt: {person, "kalle", 25}. except that if <c><anno>Name</anno></c> is a symbolic link, information about the link is returned in the <c>file_info</c> record and the <c>type</c> field of the record is set to <c>symlink</c>.</p> - <p>If the option <c>raw</c> is set, the file server is not called - and only information about local files is returned.</p> + <p>If the option <c>raw</c> is set, the file server is not called and + only information about local files is returned. Note that this will + break this module's atomicity guarantees as it can race with a + concurrent call to + <seealso marker="#write_file_info/2"><c>write_file_info/1,2</c> + </seealso></p> <p>If <c><anno>Name</anno></c> is not a symbolic link, this function returns the same result as <c>read_file_info/1</c>. On platforms that do not support symbolic links, this function @@ -2148,144 +2159,77 @@ f.txt: {person, "kalle", 25}. <section> <title>Performance</title> - <p>Some operating system file operations, for example, a - <c>sync/1</c> or <c>close/1</c> on a huge file, can block their - calling thread for seconds. If this affects the emulator main - thread, the response time is no longer in the order of - milliseconds, depending on the definition of "soft" in soft - real-time system.</p> - <p>If the device driver thread pool is active, file operations are - done through those threads instead, so the emulator can go on - executing Erlang processes. Unfortunately, the time for serving a - file operation increases because of the extra scheduling required - from the operating system.</p> - <p>If the device driver thread pool is disabled or of size 0, large - file reads and writes are segmented into many smaller, which - enable the emulator to serve other processes during the file - operation. This has the same effect as when using the thread - pool, but with larger overhead. Other file operations, for - example, <c>sync/1</c> or <c>close/1</c> on a huge file, still are - a problem.</p> - <p>For increased performance, raw files are recommended. Raw files - use the file system of the host machine of the node.</p> + <p>For increased performance, raw files are recommended.</p> + <p>A normal file is really a process so it can be used as an I/O + device (see <seealso marker="stdlib:io"><c>io</c></seealso>). + Therefore, when data is written to a normal file, the sending of the + data to the file process, copies all data that are not binaries. Opening + the file in binary mode and writing binaries is therefore recommended. + If the file is opened on another node, or if the file server runs as + slave to the file server of another node, also binaries are copied.</p> <note> - <p> - For normal files (non-raw), the file server is used to find the files, - and if the node is running its file server as slave to the file server - of another node, and the other node runs on some other host machine, - they can have different file systems. - However, this is seldom a problem.</p> + <p>Raw files use the file system of the host machine of the node. + For normal files (non-raw), the file server is used to find the files, + and if the node is running its file server as slave to the file server + of another node, and the other node runs on some other host machine, + they can have different file systems. + However, this is seldom a problem.</p> </note> - <p>A normal file is really a process so it can be used as an I/O - device (see - <seealso marker="stdlib:io"><c>io</c></seealso>). - Therefore, when data is written to a - normal file, the sending of the data to the file process, copies - all data that are not binaries. Opening the file in binary mode - and writing binaries is therefore recommended. If the file is - opened on another node, or if the file server runs as slave to - the file server of another node, also binaries are copied.</p> - <p>Caching data to reduce the number of file operations, or rather - the number of calls to the file driver, generally increases - performance. The following function writes 4 MBytes in 23 - seconds when tested:</p> + <p><seealso marker="#open/2"><c>open/2</c></seealso> can be given the + options <c>delayed_write</c> and <c>read_ahead</c> to turn on caching, + which will reduce the number of operating system calls and greatly + improve performance for small reads and writes. However, the overhead + won't disappear completely and it's best to keep the number of file + operations to a minimum. As a contrived example, the following function + writes 4MB in 2.5 seconds when tested:</p> + <code type="none"><![CDATA[ -create_file_slow(Name, N) when integer(N), N >= 0 -> - {ok, FD} = file:open(Name, [raw, write, delayed_write, binary]), - ok = create_file_slow(FD, 0, N), - ok = ?FILE_MODULE:close(FD), - ok. - -create_file_slow(FD, M, M) -> +create_file_slow(Name) -> + {ok, Fd} = file:open(Name, [raw, write, delayed_write, binary]), + create_file_slow_1(Fd, 4 bsl 20), + file:close(Fd). + +create_file_slow_1(_Fd, 0) -> ok; -create_file_slow(FD, M, N) -> - ok = file:write(FD, <<M:32/unsigned>>), - create_file_slow(FD, M+1, N).]]></code> +create_file_slow_1(Fd, M) -> + ok = file:write(Fd, <<0>>), + create_file_slow_1(Fd, M - 1).]]></code> + + <p>The following functionally equivalent code writes 128 bytes per call + to <seealso marker="#write/2"><c>write/2</c></seealso> and so does the + same work in 0.08 seconds, which is roughly 30 times faster:</p> - <p>The following, functionally equivalent, function collects 1024 - entries into a list of 128 32-byte binaries before each call to - <seealso marker="#write/2"><c>write/2</c></seealso> and so - does the same work in 0.52 seconds, - which is 44 times faster:</p> <code type="none"><![CDATA[ -create_file(Name, N) when integer(N), N >= 0 -> - {ok, FD} = file:open(Name, [raw, write, delayed_write, binary]), - ok = create_file(FD, 0, N), - ok = ?FILE_MODULE:close(FD), +create_file(Name) -> + {ok, Fd} = file:open(Name, [raw, write, delayed_write, binary]), + create_file_1(Fd, 4 bsl 20), + file:close(Fd), ok. - -create_file(FD, M, M) -> + +create_file_1(_Fd, 0) -> ok; -create_file(FD, M, N) when M + 1024 =< N -> - create_file(FD, M, M + 1024, []), - create_file(FD, M + 1024, N); -create_file(FD, M, N) -> - create_file(FD, M, N, []). - -create_file(FD, M, M, R) -> - ok = file:write(FD, R); -create_file(FD, M, N0, R) when M + 8 =< N0 -> - N1 = N0-1, N2 = N0-2, N3 = N0-3, N4 = N0-4, - N5 = N0-5, N6 = N0-6, N7 = N0-7, N8 = N0-8, - create_file(FD, M, N8, - [<<N8:32/unsigned, N7:32/unsigned, - N6:32/unsigned, N5:32/unsigned, - N4:32/unsigned, N3:32/unsigned, - N2:32/unsigned, N1:32/unsigned>> | R]); -create_file(FD, M, N0, R) -> - N1 = N0-1, - create_file(FD, M, N1, [<<N1:32/unsigned>> | R]).]]></code> +create_file_1(Fd, M) when M >= 128 -> + ok = file:write(Fd, <<0:(128)/unit:8>>), + create_file_1(Fd, M - 128); +create_file_1(Fd, M) -> + ok = file:write(Fd, <<0:(M)/unit:8>>), + create_file_1(Fd, M - 1).]]></code> - <note> - <p>Trust only your own benchmarks. If the list length in - <c>create_file/2</c> above is increased, it runs slightly - faster, but consumes more memory and causes more memory - fragmentation. How much this affects your application is - something that this simple benchmark cannot predict.</p> - <p>If the size of each binary is increased to 64 bytes, it - also runs slightly faster, but the code is then twice as clumsy. - In the current implementation, binaries larger than 64 bytes are - stored in memory common to all processes and not copied when - sent between processes, while these smaller binaries are stored - on the process heap and copied when sent like any other term.</p> - <p>So, with a binary size of 68 bytes, <c>create_file/2</c> runs - 30 percent slower than with 64 bytes, and causes much more - memory fragmentation. Notice that if the binaries were to be sent - between processes (for example, a non-raw file), the results - would probably be completely different.</p> - </note> - <p>A raw file is really a port. When writing data to a port, it is - efficient to write a list of binaries. It is not needed to - flatten a deep list before writing. On Unix hosts, scatter output, - which writes a set of buffers in one operation, is used when - possible. In this way <c>write(FD, [Bin1, Bin2 | Bin3])</c> - writes the contents of the binaries without copying the data - at all, except for perhaps deep down in the operating system - kernel.</p> - <p>For raw files, <c>pwrite/2</c> and <c>pread/2</c> are - efficiently implemented. The file driver is called only once for - the whole operation, and the list iteration is done in the file - driver.</p> - <p>The options <c>delayed_write</c> and <c>read_ahead</c> to - <seealso marker="#open/2"><c>open/2</c></seealso> - make the file driver cache data to reduce - the number of operating system calls. The function - <c>create_file/2</c> in the recent example takes 60 seconds - without option <c>delayed_write</c>, which is 2.6 - times slower.</p> - <p>As a bad example, <c>create_file_slow/2</c> - without options <c>raw</c>, <c>binary</c>, and <c>delayed_write</c>, - meaning it calls <c>open(Name, [write])</c>, needs - 1 min 20 seconds for the job, which is 3.5 times slower than - the first example, and 150 times slower than the optimized - <c>create_file/2</c>.</p> - <warning> - <p>If an error occurs when accessing an open file with module - <seealso marker="stdlib:io"><c>io</c></seealso>, - the process handling the file exits. The dead - file process can hang if a process tries to access it later. - This will be fixed in a future release.</p> - </warning> + <p>When writing data it's generally more efficient to write a list of + binaries rather than a list of integers. It is not needed to + flatten a deep list before writing. On Unix hosts, scatter output, + which writes a set of buffers in one operation, is used when + possible. In this way <c>write(FD, [Bin1, Bin2 | Bin3])</c> + writes the contents of the binaries without copying the data + at all, except for perhaps deep down in the operating system + kernel.</p> + <warning> + <p>If an error occurs when accessing an open file with module + <seealso marker="stdlib:io"><c>io</c></seealso>, the process + handling the file exits. The dead file process can hang if a process + tries to access it later. This will be fixed in a future release. + </p> + </warning> </section> <section> |