Lseek implementation Post: 302554953

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Top Forums UNIX for Dummies Questions & Answers Lseek implementation Post 302554953 by Humudituu on Tuesday 13th of September 2011 12:44:37 PM

09-13-2011

Registered User

Thank you for your replies.

Quote:

Originally Posted by Corona688

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.
(...)

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

Quote:

Originally Posted by fpmurphy

(...)

However, the behavior you see if what I would expect. Writes by their very nature are going to take longer than reads. Reads can come from cache. Writes cannot.

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
Smilie

Humudituu

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LEARN ABOUT LINUX

lseek

LSEEK(2)						     Linux Programmer's Manual							  LSEEK(2)

NAME

       lseek - reposition read/write file offset

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       off_t lseek(int fd, off_t offset, int whence);

DESCRIPTION

       The lseek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset according to the
       directive whence as follows:

       SEEK_SET
	      The offset is set to offset bytes.

       SEEK_CUR
	      The offset is set to its current location plus offset bytes.

       SEEK_END
	      The offset is set to the size of the file plus offset bytes.

       The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the file).  If  data
       is  later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes ('') until data is actually written
       into the gap.

RETURN VALUE

       Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file.	On  error,
       the value (off_t) -1 is returned and errno is set to indicate the error.

ERRORS

       EBADF  fd is not an open file descriptor.

       EINVAL whence  is  not one of SEEK_SET, SEEK_CUR, SEEK_END; or the resulting file offset would be negative, or beyond the end of a seekable
	      device.

       EOVERFLOW
	      The resulting file offset cannot be represented in an off_t.

       ESPIPE fd is associated with a pipe, socket, or FIFO.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       This document's use of whence is incorrect English, but maintained for historical reasons.

       Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

       On Linux, using lseek() on a tty device returns ESPIPE.

       When converting old code, substitute values for whence with the following macros:

	old	  new
       0	SEEK_SET
       1	SEEK_CUR
       2	SEEK_END
       L_SET	SEEK_SET
       L_INCR	SEEK_CUR
       L_XTND	SEEK_END

       Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to
       race conditions.

SEE ALSO

       dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)

COLOPHON

       This  page is part of release 3.27 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2010-09-11								  LSEEK(2)

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