Re: [PATCH] remove throttle_vm_writeout()

[Fengguang Wu]

On Thu, Oct 04, 2007 at 10:46:50AM -0700, Andrew Morton wrote:
> On Thu, 04 Oct 2007 18:47:07 +0200 Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:
> 
> > > > > > But that said, there might be better ways to do that.
> > > > > 
> > > > > Sure, if we do need to globally limit the number of under-writeback
> > > > > pages, then I think we need to do it independently of the dirty
> > > > > accounting.
> > > > 
> > > > It need not be global, it could be per BDI as well, but yes.
> > > 
> > > For per-bdi limits we have the queue length.
> > 
> > Agreed, except for:
> > 
> > static int may_write_to_queue(struct backing_dev_info *bdi)
> > {
> > 	if (current->flags & PF_SWAPWRITE)
> > 		return 1;
> > 	if (!bdi_write_congested(bdi))
> > 		return 1;
> > 	if (bdi == current->backing_dev_info)
> > 		return 1;
> > 	return 0;
> > }
> > 
> > Which will write to congested queues. Anybody know why?
> 
> 
> commit c4e2d7ddde9693a4c05da7afd485db02c27a7a09
> Author: akpm <akpm>
> Date:   Sun Dec 22 01:07:33 2002 +0000
> 
>     [PATCH] Give kswapd writeback higher priority than pdflush
>     
>     The `low latency page reclaim' design works by preventing page
>     allocators from blocking on request queues (and by preventing them from
>     blocking against writeback of individual pages, but that is immaterial
>     here).
>     
>     This has a problem under some situations.  pdflush (or a write(2)
>     caller) could be saturating the queue with highmem pages.  This
>     prevents anyone from writing back ZONE_NORMAL pages.  We end up doing
>     enormous amounts of scenning.

Sorry, I cannot not understand it. We now have balanced aging between
zones. So the page allocations are expected to distribute proportionally
between ZONE_HIGHMEM and ZONE_NORMAL?

>     A test case is to mmap(MAP_SHARED) almost all of a 4G machine's memory,
>     then kill the mmapping applications.  The machine instantly goes from
>     0% of memory dirty to 95% or more.  pdflush kicks in and starts writing
>     the least-recently-dirtied pages, which are all highmem.  The queue is
>     congested so nobody will write back ZONE_NORMAL pages.  kswapd chews
>     50% of the CPU scanning past dirty ZONE_NORMAL pages and page reclaim
>     efficiency (pages_reclaimed/pages_scanned) falls to 2%.
>     
>     So this patch changes the policy for kswapd.  kswapd may use all of a
>     request queue, and is prepared to block on request queues.
>     
>     What will now happen in the above scenario is:
>     
>     1: The page alloctor scans some pages, fails to reclaim enough
>        memory and takes a nap in blk_congetion_wait().
>     
>     2: kswapd() will scan the ZONE_NORMAL LRU and will start writing
>        back pages.  (These pages will be rotated to the tail of the
>        inactive list at IO-completion interrupt time).
>     
>        This writeback will saturate the queue with ZONE_NORMAL pages.
>        Conveniently, pdflush will avoid the congested queues.  So we end up
>        writing the correct pages.
>     
>     In this test, kswapd CPU utilisation falls from 50% to 2%, page reclaim
>     efficiency rises from 2% to 40% and things are generally a lot happier.

We may see the same problem and improvement in the absent of 'all
writeback goes to one zone' assumption.

The problem could be:
- dirty_thresh is exceeded, so balance_dirty_pages() starts syncing
  data and quickly _congests_ the queue;
- dirty pages are slowly but continuously turned into clean pages by
  balance_dirty_pages(), but they still stay in the same place in LRU;
- the zones are mostly dirty/writeback pages, kswapd has a hard time
  finding the randomly distributed clean pages;
- kswapd cannot do the writeout because the queue is congested!

The improvement could be:
- kswapd is now explicitly preferred to do the writeout;
- the pages written by kswapd will be rotated and easy for kswapd to reclaim;
- it becomes possible for kswapd to wait for the congested queue,
  instead of doing the vmscan like mad.

The congestion wait looks like a pretty natural way to throttle the kswapd.
Instead of doing the vmscan at 1000MB/s and actually freeing pages at
60MB/s(about the write throughput), kswapd will be relaxed to do vmscan at
maybe 150MB/s.

Fengguang
---
>     The downside is that kswapd may now do a lot less page reclaim,
>     increasing page allocation latency, causing more direct reclaim,
>     increasing lock contention in the VM, etc.  But I have not been able to
>     demonstrate that in testing.
>     
>     
>     The other problem is that there is only one kswapd, and there are lots
>     of disks.  That is a generic problem - without being able to co-opt
>     user processes we don't have enough threads to keep lots of disks saturated.
>     
>     One fix for this would be to add an additional "really congested"
>     threshold in the request queues, so kswapd can still perform
>     nonblocking writeout.  This gives kswapd priority over pdflush while
>     allowing kswapd to feed many disk queues.  I doubt if this will be
>     called for.
>     
>     BKrev: 3e051055aitHp3bZBPSqmq21KGs5aQ
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index c635f39..9ab0209 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -7,6 +7,7 @@ #include <linux/kdev_t.h>
>  #include <linux/linkage.h>
>  #include <linux/mmzone.h>
>  #include <linux/list.h>
> +#include <linux/sched.h>
>  #include <asm/atomic.h>
>  #include <asm/page.h>
>  
> @@ -14,6 +15,11 @@ #define SWAP_FLAG_PREFER	0x8000	/* set i
>  #define SWAP_FLAG_PRIO_MASK	0x7fff
>  #define SWAP_FLAG_PRIO_SHIFT	0
>  
> +static inline int current_is_kswapd(void)
> +{
> +	return current->flags & PF_KSWAPD;
> +}
> +
>  /*
>   * MAX_SWAPFILES defines the maximum number of swaptypes: things which can
>   * be swapped to.  The swap type and the offset into that swap type are
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index aeab1e3..a8b9d2c 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -204,6 +204,19 @@ static inline int is_page_cache_freeable
>  	return page_count(page) - !!PagePrivate(page) == 2;
>  }
>  
> +static int may_write_to_queue(struct backing_dev_info *bdi)
> +{
> +	if (current_is_kswapd())
> +		return 1;
> +	if (current_is_pdflush())	/* This is unlikely, but why not... */
> +		return 1;
> +	if (!bdi_write_congested(bdi))
> +		return 1;
> +	if (bdi == current->backing_dev_info)
> +		return 1;
> +	return 0;
> +}
> +
>  /*
>   * shrink_list returns the number of reclaimed pages
>   */
> @@ -303,8 +316,6 @@ #endif /* CONFIG_SWAP */
>  		 * See swapfile.c:page_queue_congested().
>  		 */
>  		if (PageDirty(page)) {
> -			struct backing_dev_info *bdi;
> -
>  			if (!is_page_cache_freeable(page))
>  				goto keep_locked;
>  			if (!mapping)
> @@ -313,9 +324,7 @@ #endif /* CONFIG_SWAP */
>  				goto activate_locked;
>  			if (!may_enter_fs)
>  				goto keep_locked;
> -			bdi = mapping->backing_dev_info;
> -			if (bdi != current->backing_dev_info &&
> -					bdi_write_congested(bdi))
> +			if (!may_write_to_queue(mapping->backing_dev_info))
>  				goto keep_locked;
>  			write_lock(&mapping->page_lock);
>  			if (test_clear_page_dirty(page)) {
> @@ -424,7 +433,7 @@ keep:
>  	if (pagevec_count(&freed_pvec))
>  		__pagevec_release_nonlru(&freed_pvec);
>  	mod_page_state(pgsteal, ret);
> -	if (current->flags & PF_KSWAPD)
> +	if (current_is_kswapd())
>  		mod_page_state(kswapd_steal, ret);
>  	mod_page_state(pgactivate, pgactivate);
>  	return ret;
> 

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