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Full Discussion: Lseek implementation
Top Forums UNIX for Dummies Questions & Answers Lseek implementation Post 302554953 by Humudituu on Tuesday 13th of September 2011 12:44:37 PM
Old 09-13-2011
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
 

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Benchmark::Apps(3pm)					User Contributed Perl Documentation				      Benchmark::Apps(3pm)

NAME
Benchmark::Apps - Simple interface to benchmark applications. SYNOPSIS
This module provides a simple interface to benchmark applications (not necessarily Perl applications). use Benchmark::Apps; my $commands = { cmd1 => 'run_command_1 with arguments', cmd2 => 'run_command_2 with other arguments', }; my $conf = { pretty_print=>1, iters=>5 }; Benchmark:Apps::run( $commands, $conf ); DESCRIPTION
This module can be used to perform simple benchmarks on programs. Basically, it can be used to benchmark any program that can be called with a system call. FUNCTIONS
run This method is used to run benchmarks. It runs the commands described in the hash passed as argument. It returns an hash of the results each command. A second hash reference can be passed to this method: a configuration hash reference. The values passed in this hash override the default behaviour of the run method. The configuration options available at this moment are: "pretty_print" When enabled it will print to stdout, in a formatted way the results of the benchmarks as they finish running. This option should de used when you want to run benchmarks and want to see the results progress as the tests run. You can disable it, so you can perform automated benchmarks. Options: true(1) or false(0) Default: false(0) "iters" This is the number of iterations that each test will run. Options: integer greater than 1 Default: 5 "args" This is a reference to an anonymous function that will calculate the command argument based on the iteraction number. Options: any function reference that returns a string Default: empty function: always returns an empty string, which means no arguments will be given to the command run This method runs the commands described in the hash passed as argument. It returns an hash of the results and return codes for each command. pretty_print This method is used to print the final result to STDOUT before returning from the "run" method. time_this This method is not meant to be used directly, although it can be useful. It receives a command line and executes it via system, taking care of registering the elapsed time. EXAMPLES
Check files in "examples/". AUTHOR
Aberto Simoes (aka ambs), "<ambs at cpan.org>" Nuno Carvalho (aka smash), "<smash @ cpan.org>" BUGS
Please report any bugs or feature requests to "bug-benchmark-apps at rt.cpan.org", or through the web interface at http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Benchmark-Apps <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=Benchmark-Apps>. I will be notified, and then you'll automatically be notified of progress on your bug as I make changes. SUPPORT
You can find documentation for this module with the perldoc command. perldoc Benchmark::Apps You can also look for information at: o RT: CPAN's request tracker http://rt.cpan.org/NoAuth/Bugs.html?Dist=Benchmark-Apps <http://rt.cpan.org/NoAuth/Bugs.html?Dist=Benchmark-Apps> o AnnoCPAN: Annotated CPAN documentation http://annocpan.org/dist/Benchmark-Apps <http://annocpan.org/dist/Benchmark-Apps> o CPAN Ratings http://cpanratings.perl.org/d/Benchmark-Apps <http://cpanratings.perl.org/d/Benchmark-Apps> o Search CPAN http://search.cpan.org/dist/Benchmark-Apps <http://search.cpan.org/dist/Benchmark-Apps> COPYRIGHT &; LICENSE Copyright 2008 Aberto Simoes, Nuno Carvalho, all rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. perl v5.14.2 2012-01-15 Benchmark::Apps(3pm)
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