Re: Possible bug from kernel 2.6.22 and above

["Simon Holm Thøgersen" <odie@cs.aau.dk>]

ons, 21 11 2007 kl. 20:52 -0500, skrev Jie Chen:
> Eric Dumazet wrote:
> > Jie Chen a écrit :
> >> Hi, there:
> >>
> >>     We have a simple pthread program that measures the synchronization 
> >> overheads for various synchronization mechanisms such as spin locks, 
> >> barriers (the barrier is implemented using queue-based barrier 
> >> algorithm) and so on. We have dual quad-core AMD opterons (barcelona) 
> >> clusters running 2.6.23.8 kernel at this moment using Fedora Core 7 
> >> distribution. Before we moved to this kernel, we had kernel 2.6.21. 
> >> These two kernels are configured identical and compiled with the same 
> >> gcc 4.1.2 compiler. Under the old kernel, we observed that the 
> >> performance of these overheads increases as the number of threads 
> >> increases from 2 to 8. The following are the values of total time and 
> >> overhead for all threads acquiring a pthread spin lock and all threads 
> >> executing a barrier synchronization call.
> > 
> > Could you post the source of your test program ?
> > 
> 
> 
> Hi, Eric:
> 
> Thank you for the quick response. You can get the source code containing 
> the test code from ftp://ftp.jlab.org/pub/hpc/qmt.tar.gz . This is a 
> data parallel threading package for physics calculation. The test code 
> is pthread_sync in the src directory once you unpack the gz file. To 
> configure and build this package is very simple: configure and make. The 
>   test program is built by make check. The number of threads is 
> controlled by QMT_NUM_THREADS. The package is using pthread spin lock, 
> but the barrier is implemented using a queue-based barrier algorithm 
> proposed by  J. B. Carter of University of Utah (2005).
> 
> 
> 
> 
> 
> > spinlock are ... spining and should not call linux scheduler, so I have 
> > no idea why a kernel change could modify your results.
> > 
> > Also I suspect you'll have better results with Fedora Core 8 (since 
> > glibc was updated to use private futexes in v 2.7), at least for the 
> > barrier ops.
> > 
> > 
> 
> I am not sure what the biggest change between kernel 2.6.21 and 2.6.22 
> (23) is? Is the scheduler the biggest change between these versions? Can 
> the scheduler of kernel somehow effect the performance? I know the 
> scheduler is trying to do load balance and so on. Can the scheduler move 
> threads to different cores according to the load balance algorithm even 
> though the threads are bound to cores using pthread_setaffinity_np call 
> when the number of threads is fewer than the number of cores? I am 
> thinking about this because the performance of our test code is roughly 
> the same for both kernels when the number of threads equals to the 
> number of cores.
> 
There is a backport of the CFS scheduler to 2.6.21, see
http://lkml.org/lkml/2007/11/19/127

> >>
> >> Kernel 2.6.21
> >> Number of Threads              2          4           6         8
> >> SpinLock (Time micro second)   10.5618    10.58538    10.5915   10.643
> >>                   (Overhead)   0.073      0.05746     0.102805 0.154563
> >> Barrier (Time micro second)    11.020410  11.678125   11.9889   12.38002
> >>                  (Overhead)    0.531660   1.1502      1.500112 1.891617
> >>
> >> Each thread is bound to a particular core using pthread_setaffinity_np.
> >>
> >> Kernel 2.6.23.8
> >> Number of Threads              2          4           6         8
> >> SpinLock (Time micro second)   14.849915  17.117603   14.4496   10.5990
> >>                  (Overhead)    4.345417   6.617207    3.949435  0.110985
> >> Barrier (Time micro second)    19.462255  20.285117   16.19395  12.37662
> >>                  (Overhead)    8.957755   9.784722    5.699590  1.869518
> >>
> >> It is clearly that the synchronization overhead increases as the 
> >> number of threads increases in the kernel 2.6.21. But the 
> >> synchronization overhead actually decreases as the number of threads 
> >> increases in the kernel 2.6.23.8 (We observed the same behavior on 
> >> kernel 2.6.22 as well). This certainly is not a correct behavior. The 
> >> kernels are configured with CONFIG_SMP, CONFIG_NUMA, CONFIG_SCHED_MC, 
> >> CONFIG_PREEMPT_NONE, CONFIG_DISCONTIGMEM set. The complete kernel 
> >> configuration file is in the attachment of this e-mail.
> >>
> >>  From what we have read, there was a new scheduler (CFS) appeared from 
> >> 2.6.22. We are not sure whether the above behavior is caused by the 
> >> new scheduler.
> >>
> >> Finally, our machine cpu information is listed in the following:
> >>
> >> processor       : 0
> >> vendor_id       : AuthenticAMD
> >> cpu family      : 16
> >> model           : 2
> >> model name      : Quad-Core AMD Opteron(tm) Processor 2347
> >> stepping        : 10
> >> cpu MHz         : 1909.801
> >> cache size      : 512 KB
> >> physical id     : 0
> >> siblings        : 4
> >> core id         : 0
> >> cpu cores       : 4
> >> fpu             : yes
> >> fpu_exception   : yes
> >> cpuid level     : 5
> >> wp              : yes
> >> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge 
> >> mca cmov
> >> pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt 
> >> pdpe1gb rdtscp
> >>  lm 3dnowext 3dnow constant_tsc rep_good pni cx16 popcnt lahf_lm 
> >> cmp_legacy svm
> >> extapic cr8_legacy altmovcr8 abm sse4a misalignsse 3dnowprefetch osvw
> >> bogomips        : 3822.95
> >> TLB size        : 1024 4K pages
> >> clflush size    : 64
> >> cache_alignment : 64
> >> address sizes   : 48 bits physical, 48 bits virtual
> >> power management: ts ttp tm stc 100mhzsteps hwpstate
> >>
> >> In addition, we have schedstat and sched_debug files in the /proc 
> >> directory.
> >>
> >> Thank you for all your help to solve this puzzle. If you need more 
> >> information, please let us know.
> >>
> >>
> >> P.S. I like to be cc'ed on the discussions related to this problem.
> >>
> 
> Thank you for your help and happy thanksgiving !
> 


Simon Holm Thøgersen