linux-kernel - Re: I/O and pdflush

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Date:	Sat, 29 Aug 2009 18:21:26 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Fernando Silveira <fsilveira@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: I/O and pdflush

On Sat, Aug 29, 2009 at 06:12:47PM +0800, Wu Fengguang wrote:
> On Sat, Aug 29, 2009 at 05:48:40AM +0800, Fernando Silveira wrote:
> > On Sun, Jul 12, 2009 at 05:04, Wu Fengguang<fengguang.wu@...el.com> wrote:
> > > On Sat, Jul 11, 2009 at 02:27:25PM -0300, Fernando Silveira wrote:
> > >> I'm having a hard time with an application that writes sequentially
> > >> 250GB of non-stop data directly to a solid state disk (OCZ SSD CORE
> > >> v2) device and I hope you can help me. The command "dd if=/dev/zero
> > >> of=/dev/sdc bs=4M" reproduces the same symptoms I'm having and writes
> > >> exactly as that application does.
> > >
> > > What's your kernel version?  Can the following patch help?
> > 
> > Sorry for the delay, but I could not test it before.
> > 
> > Unfortunately it did not help or change any symptoms at all. The
> > kernel version I tested was a non-patched 2.6.30.1 version.
> > 
> > Do you have any other hint?
> 
> Sure. Attached is a writeback debug patch. It will generate lots of
> kernel messages. You can just stop your klogd, start your workload
> and monitor the SSD writeback throughput via tools like iostat/dstat.

Fernando, will you post the output of iostat/dstat too? Thanks!
The ssd-no_dirty_buffer_with_random_192mb_writes.png is a good overview,
however I'd like to also check out the numbers for each second :)

Thanks,
Fengguang

> When it goes into the suboptimal 25MB/s state for several seconds, run
>         dmesg > dmesg-writeback
> and send me the log.
> 
> And it is advised to turn on the kconfig option CONFIG_PRINTK_TIME=y.
> 
> Thanks,
> Fengguang

>  mm/page-writeback.c |   38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> --- mm.orig/mm/page-writeback.c
> +++ mm/mm/page-writeback.c
> @@ -116,6 +116,33 @@ EXPORT_SYMBOL(laptop_mode);
>  
>  /* End of sysctl-exported parameters */
>  
> +#define writeback_debug_report(n, wbc) do {                               \
> +	__writeback_debug_report(n, wbc, __FILE__, __LINE__, __FUNCTION__); \
> +} while (0)
> +
> +void print_writeback_control(struct writeback_control *wbc)
> +{
> +	printk(KERN_DEBUG
> +			"global dirty %lu writeback %lu nfs %lu "
> +			"flags %c%c towrite %ld skipped %ld\n",
> +			global_page_state(NR_FILE_DIRTY),
> +			global_page_state(NR_WRITEBACK),
> +			global_page_state(NR_UNSTABLE_NFS),
> +			wbc->encountered_congestion ? 'C':'_',
> +			wbc->more_io ? 'M':'_',
> +			wbc->nr_to_write,
> +			wbc->pages_skipped);
> +}
> +
> +void __writeback_debug_report(long n, struct writeback_control *wbc,
> +		const char *file, int line, const char *func)
> +{
> +	printk(KERN_DEBUG "%s %d %s: %s(%d) %ld\n",
> +			file, line, func,
> +			current->comm, current->pid,
> +			n);
> +	print_writeback_control(wbc);
> +}
>  
>  static void background_writeout(unsigned long _min_pages);
>  
> @@ -546,6 +573,7 @@ static void balance_dirty_pages(struct a
>  			pages_written += write_chunk - wbc.nr_to_write;
>  			get_dirty_limits(&background_thresh, &dirty_thresh,
>  				       &bdi_thresh, bdi);
> +			writeback_debug_report(pages_written, &wbc);
>  		}
>  
>  		/*
> @@ -572,6 +600,7 @@ static void balance_dirty_pages(struct a
>  			break;		/* We've done our duty */
>  
>  		congestion_wait(WRITE, HZ/10);
> +		writeback_debug_report(-pages_written, &wbc);
>  	}
>  
>  	if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
> @@ -666,6 +695,11 @@ void throttle_vm_writeout(gfp_t gfp_mask
>  			global_page_state(NR_WRITEBACK) <= dirty_thresh)
>                          	break;
>                  congestion_wait(WRITE, HZ/10);
> +		printk(KERN_DEBUG "throttle_vm_writeout: "
> +				"congestion_wait on %lu+%lu > %lu\n",
> +				global_page_state(NR_UNSTABLE_NFS),
> +				global_page_state(NR_WRITEBACK),
> +				dirty_thresh);
>  
>  		/*
>  		 * The caller might hold locks which can prevent IO completion
> @@ -715,7 +749,9 @@ static void background_writeout(unsigned
>  			else
>  				break;
>  		}
> +		writeback_debug_report(min_pages, &wbc);
>  	}
> +	writeback_debug_report(min_pages, &wbc);
>  }
>  
>  /*
> @@ -788,7 +824,9 @@ static void wb_kupdate(unsigned long arg
>  				break;	/* All the old data is written */
>  		}
>  		nr_to_write -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
> +		writeback_debug_report(nr_to_write, &wbc);
>  	}
> +	writeback_debug_report(nr_to_write, &wbc);
>  	if (time_before(next_jif, jiffies + HZ))
>  		next_jif = jiffies + HZ;
>  	if (dirty_writeback_interval)

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