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 DEBIAN

go-testflag

GO-TESTFLAG(7)						 Miscellaneous Information Manual					    GO-TESTFLAG(7)

NAME

       go - tool for managing Go source code

DESCRIPTION

       The 'go test' command takes both flags that apply to 'go test' itself and flags that apply to the resulting test binary.

       The test binary, called pkg.test, where pkg is the name of the directory containing the package sources, has its own flags:

       -test.v
	      Verbose output: log all tests as they are run.

       -test.run pattern
	      Run only those tests and examples matching the regular expression.

       -test.bench pattern
	      Run benchmarks matching the regular expression.  By default, no benchmarks run.

       -test.cpuprofile cpu.out
	      Write a CPU profile to the specified file before exiting.

       -test.memprofile mem.out
	      Write a memory profile to the specified file when all tests are complete.

       -test.memprofilerate n
	      Enable more precise (and expensive) memory profiles by setting runtime.MemProfileRate.  See 'godoc runtime MemProfileRate'.  To pro-
	      file all memory allocations, use -test.memprofilerate=1 and set the environment variable GOGC=off to disable the garbage	collector,
	      provided the test can run in the available memory without garbage collection.

       -test.parallel n
	      Allow  parallel  execution  of  test  functions  that call t.Parallel.  The value of this flag is the maximum number of tests to run
	      simultaneously; by default, it is set to the value of GOMAXPROCS.

       -test.short
	      Tell long-running tests to shorten their run time.  It is off by default but set during all.bash so that installing the Go tree  can
	      run a sanity check but not spend time running exhaustive tests.

       -test.timeout t
	      If a test runs longer than t, panic.

       -test.benchtime n
	      Run enough iterations of each benchmark to take n seconds.  The default is 1 second.

       -test.cpu 1,2,4
	      Specify  a  list	of  GOMAXPROCS	values	for which the tests or benchmarks should be executed.  The default is the current value of
	      GOMAXPROCS.

       For convenience, each of these -test.X flags of the test binary is also available as the flag -X in 'go test'  itself.	Flags  not  listed
       here are passed through unaltered.  For instance, the command

	     go test -x -v -cpuprofile=prof.out -dir=testdata -update

       will compile the test binary and then run it as

	     pkg.test -test.v -test.cpuprofile=prof.out -dir=testdata -update

AUTHOR

       This manual page was written by Michael Stapelberg <stapelberg@debian.org>, for the Debian project (and may be used by others).

								    2012-05-13							    GO-TESTFLAG(7)

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