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SubjectRe: CFQ is worse than other IO schedulers in some cases
On Wed, Feb 18 2009, Shan Wei wrote:
> I found that CFQ's performance is worse than other IO scheduer in some cases
> I confirmed its phenomenon when I executed dump command and sysbench on 2.6.28.
>
>
> In dump(version:dump-0.4b41-2.fc6), I confirmed
> the speed under CFQ is slower than other IO schedulers.
>
>
> The Test Result(dump):
> UNIT:Mb/sec
> _______________________
> | IO | |
> | scheduler | Speed |
> +------------|--------|
> |cfq | 24.310 |
> |noop | 36.885 |
> |anticipatory| 34.956 |
> |deadline | 36.758 |
> +----------------------
>
>
> Steps to reproduce(dump):
> #dump -0uf /dev/null /dev/sda6

The dump issue is a known one, it has to do with how dump uses seperate
processes to interleave IO to the 'same' location. Jeff Moyer posted a
fix for that some time ago, you can also find references to the
discussion and progress right here on lkml. For reference, patch is
included.

> In sysbench(version:sysbench-0.4.10), I confirmed followings.
> - CFQ's performance is worse than other IO schedulers when only multiple
> threads test.
> (There is no difference under single thread test.)
> - It is worse than other IO scheduler when
> I used read mode. (No regression in write mode).
> - There is no difference among other IO schedulers. (e.g noop deadline)
>
>
> The Test Result(sysbench):
> UNIT:Mb/sec
> __________________________________________________
> | IO | thread number |
> | scheduler |-----------------------------------|
> | | 1 | 3 | 5 | 7 | 9 |
> +------------|------|-------|------|------|------|
> |cfq | 77.8 | 32.4 | 43.3 | 55.8 | 58.5 |
> |noop | 78.2 | 79.0 | 78.2 | 77.2 | 77.0 |
> |anticipatory| 78.2 | 78.6 | 78.4 | 77.8 | 78.1 |
> |deadline | 76.9 | 78.4 | 77.0 | 78.4 | 77.9 |
> +------------------------------------------------+

What kind of storage hardware did you use?

------

Hi,

dump performs poorly when run under the CFQ I/O scheduler. The reason
for this is that the dump command interleaves I/O between two (or
three?) cooperating processes. This is about the worst case scenario
you can get for CFQ, as the I/O access pattern within each process is
sequential. Thus, CFQ will idle for a number of milliseconds waiting
for the current process to issue more I/O before switching to the next.

Now, this behaviour can be changed with tuning. However, if the dump
command simply shared I/O contexts between cooperating processes, CFQ
could make more intelligent decisions about I/O scheduling.

So, here are the numbers, running under 2.6.28-rc3.

deadline 82241 kB/s
cfq 34143 kB/s
cfq-shared 82241 kB/s

cfq-shared denotes that the dump utility was patched with the attached
patch to share I/O contexts. As you can see, with a very little bit of
code change, we can drastically increase the performance of dump under
CFQ (which is the default I/O scheduler used in a number of
distributions).

For more information on the underlying problems, you can refer to the
following kernel discussion:
http://lkml.org/lkml/2008/11/9/133

Comments are appreciated.

Cheers,

Jeff

diff -up ./dump/tape.c.orig ./dump/tape.c
--- ./dump/tape.c.orig 2005-08-20 17:00:48.000000000 -0400
+++ ./dump/tape.c 2008-11-17 16:40:42.575792509 -0500
@@ -187,6 +187,40 @@ static sigjmp_buf jmpbuf; /* where to ju
static int gtperr = 0;
#endif

+/*
+ * Determine if we can use Linux' clone system call. If so, call it
+ * with the CLONE_IO flag so that all processes will share the same I/O
+ * context, allowing the I/O schedulers to make better scheduling decisions.
+ */
+#ifdef __linux__
+#include <syscall.h>
+
+#ifndef SYS_clone
+#define fork_clone_io fork
+#else /* SYS_clone */
+#include <linux/version.h>
+
+/*
+ * Kernel 2.5.49 introduced two extra parameters to the clone system call.
+ * Neither is useful in our case, so this is easy to handle.
+ */
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,5,49)
+/* clone_flags, child_stack, parent_tidptr, child_tidptr */
+#define CLONE_ARGS SIGCHLD|CLONE_IO, 0, NULL, NULL
+#else
+#define CLONE_ARGS SIGCHLD|CLONE_IO, 0
+#endif /* LINUX_VERSION_CODE */
+
+#define _GNU_SOURCE
+#include <sched.h>
+#include <unistd.h>
+#undef _GNU_SOURCE
+pid_t fork_clone_io(void);
+#endif /* SYS_clone */
+#else /* __linux__ not defined */
+#define fork_clone_io fork
+#endif /* __linux__ */
+
int
alloctape(void)
{
@@ -755,6 +789,16 @@ rollforward(void)
#endif
}

+#ifdef __linux__
+#ifdef SYS_clone
+pid_t
+fork_clone_io(void)
+{
+ return syscall(SYS_clone, CLONE_ARGS);
+}
+#endif
+#endif
+
/*
* We implement taking and restoring checkpoints on the tape level.
* When each tape is opened, a new process is created by forking; this
@@ -801,7 +845,7 @@ restore_check_point:
/*
* All signals are inherited...
*/
- childpid = fork();
+ childpid = fork_clone_io();
if (childpid < 0) {
msg("Context save fork fails in parent %d\n", parentpid);
Exit(X_ABORT);
@@ -1017,7 +1061,7 @@ enslave(void)
}

if (socketpair(AF_UNIX, SOCK_STREAM, 0, cmd) < 0 ||
- (slaves[i].pid = fork()) < 0)
+ (slaves[i].pid = fork_clone_io()) < 0)
quit("too many slaves, %d (recompile smaller): %s\n",
i, strerror(errno));

--
Jens Axboe



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