Re: CONFIG_PREEMPT and server workloads

[Takashi Iwai]

At Thu, 18 Mar 2004 14:48:07 -0500,
Chris Mason wrote:
> 
> On Thu, 2004-03-18 at 14:29, Andrew Morton wrote:
> 
> > > yep, i see a similar problem also in reiserfs's do_journal_end().
> > > it's in lock_kernel().
> > 
> > I have a scheduling point in journal_end() in 2.4.  But I added bugs to
> > reiserfs a couple of times doing this - it's pretty delicate.  Beat up on
> > Chris ;)
> 
> ;-) Not sure if Takashi is talking about -suse or -mm, the data=ordered
> patches change things around.  He sent me suggestions for the
> data=ordered latencies already, but it shouldn't be against the BKL
> there, since I drop it before calling write_ordered_buffers().

i tested only suse kernels recently.  will try mm kernel later, again.

ok, let me explain some nasty points i found through the disk i/o load
tests:

- in the loop in do_journal_end().  this happens periodically in
  pdflush.

  /* first data block is j_start + 1, so add one to cur_write_start wherever you use it */
  cur_write_start = SB_JOURNAL(p_s_sb)->j_start ;
  cn = SB_JOURNAL(p_s_sb)->j_first ;
  jindex = 1 ; /* start at one so we don't get the desc again */
  while(cn) {
    clear_bit(BH_JNew, &(cn->bh->b_state)) ;
    ....
    next = cn->next ;
    free_cnode(p_s_sb, cn) ;
    cn = next ;
  }


- in write_ordered_buffers().

  i still don't figure out where.  we have already cond_resched()
  check in the loops.  this one is triggered when i write bulk data
  in parallel (1GB write with 20 threads at the same time), resulting
  in up to 2ms.

  a typical stacktracing looks like this:

	T=36.569 diff=3.64275
	  comm=reiserfs/0
	  rtc_interrupt (+cd/e0)
	  handle_IRQ_event (+2f/60)
	  do_IRQ (+76/170)
	  common_interrupt (+18/20)
	  kfree (+36/50)
	  reiserfs_free_jh (+34/60)
	  write_ordered_buffers (+11f/1d0)
	  flush_commit_list (+3e6/480)
	  flush_async_commits (+5d/70)
	  worker_thread (+164/1d0)
	  flush_async_commits (+0/70)
	  default_wake_function (+0/10)
	  default_wake_function (+0/10)
	  worker_thread (+0/1d0)
	  kthread (+77/9f)
	  kthread (+0/9f)
	  kernel_thread_helper (+5/10)


Takashi
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