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Full Discussion: Lseek implementation
Top Forums UNIX for Dummies Questions & Answers Lseek implementation Post 302554895 by Corona688 on Tuesday 13th of September 2011 10:00:03 AM
Old 09-13-2011
ext4 uses generic_file_llseek for lseek, and I find this implementation for that in fs/read_write.c:
Code:
/**
 * generic_file_llseek - generic llseek implementation for regular files
 * @file:       file structure to seek on
 * @offset:     file offset to seek to
 * @origin:     type of seek
 *
 * This is a generic implemenation of ->llseek useable for all normal local
 * filesystems.  It just updates the file offset to the value specified by
 * @offset and @origin under i_mutex.
 */
loff_t generic_file_llseek(struct file *file, loff_t offset, int origin)
{
        loff_t rval;

        mutex_lock(&file->f_dentry->d_inode->i_mutex);
        rval = generic_file_llseek_unlocked(file, offset, origin);
        mutex_unlock(&file->f_dentry->d_inode->i_mutex);

        return rval;
}

/**
 * generic_file_llseek_unlocked - lockless generic llseek implementation
 * @file:       file structure to seek on
 * @offset:     file offset to seek to
 * @origin:     type of seek
 *
 * Updates the file offset to the value specified by @offset and @origin.
 * Locking must be provided by the caller.
 */
loff_t
generic_file_llseek_unlocked(struct file *file, loff_t offset, int origin)
{
        struct inode *inode = file->f_mapping->host;

        switch (origin) {
        case SEEK_END:
                offset += inode->i_size;
                break;
        case SEEK_CUR:
                /*
                 * Here we special-case the lseek(fd, 0, SEEK_CUR)
                 * position-querying operation.  Avoid rewriting the "same"
                 * f_pos value back to the file because a concurrent read(),
                 * write() or lseek() might have altered it
                 */
                if (offset == 0)
                        return file->f_pos;
               break;
        }

        if (offset < 0 || offset > inode->i_sb->s_maxbytes)
                return -EINVAL;

        /* Special lock needed here? */
        if (offset != file->f_pos) {
                file->f_pos = offset;

                file->f_version = 0;
        }

        return offset;
}

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 don't think this'd happen if you hadn't opened it with O_DIRECT. Caching is your friend...
Last edited by Corona688; 09-13-2011 at 11:05 AM..
 

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LEARN ABOUT CENTOS

pmdatrivial

PMDATRIVIAL(1)						      General Commands Manual						    PMDATRIVIAL(1)

NAME

       pmdatrivial - trivial performance metrics domain agent (PMDA)

SYNOPSIS

       $PCP_PMDAS_DIR/trivial/pmdatrivial [-d domain] [-l logfile] [-U username]

DESCRIPTION

       pmdatrivial is the simplest possible Performance Metrics Domain Agent (PMDA) which exports a single performance metric, the time in seconds
       since the 1st of January, 1970.

       The trivial PMDA is shipped as source code and is designed to be an aid for PMDA developers.

       A brief description of the pmdatrivial command line options follows:

       -d   It is absolutely crucial that the performance metrics domain number specified here is unique and consistent.  That is,  domain  should
	    be different for every PMDA on the one host, and the same domain number should be used for the same PMDA on all hosts.

       -l   Location of the log file.  By default, a log file named trivial.log is written in the current directory of pmcd(1) when pmdatrivial is
	    started, i.e.  $PCP_LOG_DIR/pmcd.  If the log file cannot be created or is not writable, output  is  written  to  the  standard  error
	    instead.

       -U   User  account  under  which  to run the agent.  The default is the unprivileged "pcp" account in current versions of PCP, but in older
	    versions the superuser account ("root") was used by default.

INSTALLATION

       If you want access to the names, help text and values for the trivial performance metrics, do the following as root:

	    # cd $PCP_PMDAS_DIR/trivial
	    # ./Install

       If you want to undo the installation, do the following as root:

	    # cd $PCP_PMDAS_DIR/trivial
	    # ./Remove

       pmdatrivial is launched by pmcd(1) and should never be executed directly.  The Install and Remove scripts notify pmcd(1) when the agent	is
       installed or removed.

FILES

       $PCP_PMCDCONF_PATH
		 command line options used to launch pmdatrivial
       $PCP_PMDAS_DIR/trivial/help
		 default help text file for the trivial metrics
       $PCP_PMDAS_DIR/trivial/Install
		 installation script for the pmdatrivial agent
       $PCP_PMDAS_DIR/trivial/Remove
		 undo installation script for the pmdatrivial agent
       $PCP_LOG_DIR/pmcd/trivial.log
		 default log file for error messages and other information from pmdatrivial

PCP ENVIRONMENT

       Environment  variables  with  the prefix PCP_ are used to parameterize the file and directory names used by PCP.  On each installation, the
       file /etc/pcp.conf contains the local values for these variables.  The $PCP_CONF variable may be used to specify an alternative	configura-
       tion file, as described in pcp.conf(5).

SEE ALSO

       PCPIntro(1), pmcd(1), pmdasample(1), pmdasimple(1), pmdatxmon(1) and PMDA(3).

Performance Co-Pilot							PCP							    PMDATRIVIAL(1)
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