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UNIX for Dummies Questions & Answers
Lseek implementation
Post 302554953 by Humudituu on Tuesday 13th of September 2011 12:44:37 PM
09-13-2011
Thank you for your replies.
I'm trying to wrap my mind around this... The mutex should be released after the lseek, right? Is the mutex active while writing? Otherwise the behaviour explanied below wouldn't make sense to me, as either lseek while reading would be slow as well or the mutex should be released rather quickly... :S
I would hardly believe this statement to be generally true as writes can be asynchronous, but that is another story.
The point is that I'm having huge lseek latencies when running a benchmark where 100 threads are writing randomly into files compared to 100 threads randomly reading files:
a) read, lseek, read, lseek, read, lseek,...
mean read latency: ~4ms
mean lseek latency: ~0,001ms
b) write, lseek, write, lseek, ...
mean write latency: ~10ms
mean lseek latency: ~8ms
Quote:
ext4 uses generic_file_llseek for lseek, and I find this implementation for that in fs/read_write.c:
(...)
So really, nothing to it, and the only thing that could be blocking is that mutex...
I think you've saturated the kernel with so many simultaneous system calls to the same inode that they're competing for i_mutex.
(...)
(...)
So really, nothing to it, and the only thing that could be blocking is that mutex...
I think you've saturated the kernel with so many simultaneous system calls to the same inode that they're competing for i_mutex.
(...)
Quote:
The point is that I'm having huge lseek latencies when running a benchmark where 100 threads are writing randomly into files compared to 100 threads randomly reading files:
a) read, lseek, read, lseek, read, lseek,...
mean read latency: ~4ms
mean lseek latency: ~0,001ms
b) write, lseek, write, lseek, ...
mean write latency: ~10ms
mean lseek latency: ~8ms
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LEARN ABOUT OSF1
lseek
lseek(2) System Calls Manual lseek(2) NAME
lseek - Moves read-write file offset SYNOPSIS
#include <sys/types.h> #include <unistd.h> off_t lseek ( int filedes, off_t offset, int whence ); STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: lseek(): XSH5.0, XNS5.0 Refer to the standards(5) reference page for more information about industry standards and associated tags. PARAMETERS
Specifies a file descriptor obtained from a successful open() or fcntl() function. Specifies a value, in bytes, that is used in conjunc- tion with the whence parameter to set the file pointer. A negative value causes seeking in the reverse direction. The resulting file posi- tion may also be negative. Specifies how to interpret the offset parameter in setting the file pointer associated with the filedes parame- ter. Values for the whence parameter are: Sets the file pointer to the value of the offset parameter. Sets the file pointer to its cur- rent location plus the value of the offset parameter. Sets the file pointer to the size of the file plus the value of the offset parame- ter. DESCRIPTION
The lseek() function sets the file offset for the open file specified by the filedes parameter. The whence parameter determines how the offset is to be interpreted. The lseek() function allows the file offset to be set beyond the end of existing data in the file. If data is later written at this point, subsequently reading data in the gap returns bytes with the value 0 (zero) until data is actually written into the gap. By itself, the lseek() function does not extend the size of the file. RETURN VALUES
Upon successful completion, the resulting pointer location (the file offset), measured in bytes from the beginning of the file, is returned. If the lseek() function fails, the file offset remains unchanged, a value of (off_t) - 1 is returned, and errno is set to indi- cate the error. ERRORS
If the lseek() function fails, the file offset remains unchanged and errno may be set to one of the following values: The filedes parameter is not an open file descriptor. The whence parameter is an invalid value or the resulting file offset would be invalid. The filedes parameter is associated with a pipe (FIFO), a socket, or a multiplexed special file. The filedes parameter underlying the stream is associated with a socket. RELATED INFORMATION
Functions: fcntl(2), fseek(3), open(2), read(2), write(2) Standards: standards(5) delim off lseek(2)