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Date: Wed, 26 Sep 2007 15:31:38 +0200
From: Ingo Molnar <mingo@...e.hu>
To: David Schwartz <davids@...master.com>
Cc: "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
Mike Galbraith <efault@....de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Martin Michlmayr <tbm@...ius.com>,
Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
Stephen Hemminger <shemminger@...ux-foundation.org>
Subject: Re: Network slowdown due to CFS
* David Schwartz <davids@...master.com> wrote:
> > > I think the real fix would be for iperf to use blocking network IO
> > > though, or maybe to use a POSIX mutex or POSIX semaphores.
> >
> > So it's definitely not a bug in the kernel, only in iperf?
>
> Martin:
>
> Actually, in this case I think iperf is doing the right thing (though not
> the best thing) and the kernel is doing the wrong thing. [...]
it's not doing the right thing at all. I had a quick look at the source
code, and the reason for that weird yield usage was that there's a
locking bug in iperf's "Reporter thread" abstraction and apparently
instead of fixing the bug it was worked around via a horrible yield()
based user-space lock.
the (small) patch below fixes the iperf locking bug and removes the
yield() use. There are numerous immediate benefits of this patch:
- iperf uses _much_ less CPU time. On my Core2Duo test system, before
the patch it used up 100% CPU time to saturate 1 gigabit of network
traffic to another box. With the patch applied it now uses 9% of
CPU time.
- sys_sched_yield() is removed altogether
- i was able to measure much higher bandwidth over localhost for
example. This is the case for over-the-network measurements as well.
- the results are also more consistent and more deterministic, hence
more reliable as a benchmarking tool. (the reason for that is that
more CPU time is spent on actually delivering packets, instead of
mindlessly polling on the user-space "lock", so we actually max out
the CPU, instead of relying on the random proportion the workload was
able to make progress versus wasting CPU time on polling.)
sched_yield() is almost always the symptom of broken locking or other
bug. In that sense CFS does the right thing by exposing such bugs =B-)
Ingo
------------------------->
Subject: iperf: fix locking
From: Ingo Molnar <mingo@...e.hu>
fix iperf locking - it was burning CPU time while polling
unnecessarily, instead of using the proper wait primitives.
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
compat/Thread.c | 3 ---
src/Reporter.c | 13 +++++++++----
src/main.cpp | 2 ++
3 files changed, 11 insertions(+), 7 deletions(-)
Index: iperf-2.0.2/compat/Thread.c
===================================================================
--- iperf-2.0.2.orig/compat/Thread.c
+++ iperf-2.0.2/compat/Thread.c
@@ -405,9 +405,6 @@ int thread_numuserthreads( void ) {
void thread_rest ( void ) {
#if defined( HAVE_THREAD )
#if defined( HAVE_POSIX_THREAD )
- // TODO add checks for sched_yield or pthread_yield and call that
- // if available
- usleep( 0 );
#else // Win32
SwitchToThread( );
#endif
Index: iperf-2.0.2/src/Reporter.c
===================================================================
--- iperf-2.0.2.orig/src/Reporter.c
+++ iperf-2.0.2/src/Reporter.c
@@ -111,6 +111,7 @@ report_statistics multiple_reports[kRepo
char buffer[64]; // Buffer for printing
ReportHeader *ReportRoot = NULL;
extern Condition ReportCond;
+extern Condition ReportDoneCond;
int reporter_process_report ( ReportHeader *report );
void process_report ( ReportHeader *report );
int reporter_handle_packet( ReportHeader *report );
@@ -338,7 +339,7 @@ void ReportPacket( ReportHeader* agent,
// item
while ( index == 0 ) {
Condition_Signal( &ReportCond );
- thread_rest();
+ Condition_Wait( &ReportDoneCond );
index = agent->reporterindex;
}
agent->agentindex = 0;
@@ -346,7 +347,7 @@ void ReportPacket( ReportHeader* agent,
// Need to make sure that reporter is not about to be "lapped"
while ( index - 1 == agent->agentindex ) {
Condition_Signal( &ReportCond );
- thread_rest();
+ Condition_Wait( &ReportDoneCond );
index = agent->reporterindex;
}
@@ -553,6 +554,7 @@ void reporter_spawn( thread_Settings *th
}
Condition_Unlock ( ReportCond );
+again:
if ( ReportRoot != NULL ) {
ReportHeader *temp = ReportRoot;
//Condition_Unlock ( ReportCond );
@@ -575,9 +577,12 @@ void reporter_spawn( thread_Settings *th
// finished with report so free it
free( temp );
Condition_Unlock ( ReportCond );
+ Condition_Signal( &ReportDoneCond );
+ if (ReportRoot)
+ goto again;
}
- // yield control of CPU is another thread is waiting
- thread_rest();
+ Condition_Signal( &ReportDoneCond );
+ usleep(10000);
} else {
//Condition_Unlock ( ReportCond );
}
Index: iperf-2.0.2/src/main.cpp
===================================================================
--- iperf-2.0.2.orig/src/main.cpp
+++ iperf-2.0.2/src/main.cpp
@@ -96,6 +96,7 @@ extern "C" {
// records being accessed in a report and also to
// serialize modification of the report list
Condition ReportCond;
+ Condition ReportDoneCond;
}
// global variables only accessed within this file
@@ -141,6 +142,7 @@ int main( int argc, char **argv ) {
// Initialize global mutexes and conditions
Condition_Initialize ( &ReportCond );
+ Condition_Initialize ( &ReportDoneCond );
Mutex_Initialize( &groupCond );
Mutex_Initialize( &clients_mutex );
-
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