Re: [PATCH -mm] make swapin readahead skip over holes

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Jan 11, 2012 at 02:23:58PM -0500, Rik van Riel wrote:
> On 01/11/2012 11:51 AM, Mel Gorman wrote:
> >On Mon, Jan 09, 2012 at 06:10:23PM -0500, Rik van Riel wrote:
> 
> >>+	get_swap_cluster(entry,&offset,&end_offset);
> >>+
> >>+	for (; offset<= end_offset ; offset++) {
> >>  		/* Ok, do the async read-ahead now */
> >>  		page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
> >>  						gfp_mask, vma, addr);
> >>  		if (!page)
> >>-			break;
> >>+			continue;
> >>  		page_cache_release(page);
> >>  	}
> >
> >For a heavily fragmented swap file, this will result in more IO and
> >the gamble is that pages nearby are needed soon. You say your virtual
> >machines swapin faster and that does not surprise me. I also expect
> >they need the data so it's a net win.
> 
> More IO operations, yes.  However, IO operations from nearby
> blocks can often be done without incurring extra disk seeks,
> because the disk head is already in the right place.
> 

I understand that, the result still has to go somewhere in memory
though. The cost in terms of IO speed might be effectively 0 but it
still consumes memory resources.

> This seems to be born out by the fact that I saw swapin
> rates increase from maybe 200-300kB/s to 5-15MB/s...
> 
> Even on some SSDs it could avoid some bank switches, though
> of course there I would expect the effect to be much less
> pronounced.
> 
> >There is an possibility that under memory pressure that swapping in
> >more pages will cause more memory pressure (increased swapin causing
> >clean page cache discards and pageout) and be an overall loss. This may
> >be a net loss in some cases such as where the working set size is just
> >over physical memory and the increased swapin causes a problem. I doubt
> >this case is common but it is worth bearing in mind if future bug
> >reports complain about increased swap activity.
> 
> True, there may be workloads that benefit from a smaller
> page-cluster. The fact that the recently swapped in pages
> are all put on the inactive anon list should help protect
> the working set, too.
> 

True to some extent.

> Another alternative may be a time based decision. If we
> have swapped something out recently, go with a less
> aggressive swapin readahead.
> 

Maybe, but while I know I brought up the problem of swapin might
increase swapout, it would be best to have an example of how that can
happen before looking at heuristics.

> That would automatically give us fast swapin readahead
> when in "memory hog just exited, let the system recover"
> mode, and conservative swapin readahead in your situation.
> 
> However, that could still hurt badly if the system is just
> moving the working set from one part of a program to another.
> 
> I suspect we will be faster off by having faster swap IO,
> which this patch seems to provide.
> 

More than likely.

> >>-	si = swap_info[swp_type(entry)];
> >>-	target = swp_offset(entry);
> >>-	base = (target>>  our_page_cluster)<<  our_page_cluster;
> >>-	end = base + (1<<  our_page_cluster);
> >>-	if (!base)		/* first page is swap header */
> >>-		base++;
> 
> >>+	si = swap_info[swp_type(entry)];
> >>+	/* Round the begin down to a page_cluster boundary. */
> >>+	offset = (offset>>  page_cluster)<<  page_cluster;
> >
> >Minor nit but it would feel more natural to me to see
> >
> >offset&  ~((1<<  page_cluster) - 1)
> >
> >but I understand that you are reusing the existing code.
> 
> Sure, I can do that.
> 
> While I'm there, I can also add that if (!base) base++
> thing back in :)
> 

Whoops, I missed that!

> >>+	*begin = offset;
> >>+	/* Round the end up, but not beyond the end of the swap device. */
> >>+	offset = offset + (1<<  page_cluster);
> >>+	if (offset>  si->max)
> >>+		offset = si->max;
> >>+	*end = offset;
> >>  	spin_unlock(&swap_lock);
> >>-
> >>-	/*
> >>-	 * Indicate starting offset, and return number of pages to get:
> >>-	 * if only 1, say 0, since there's then no readahead to be done.
> >>-	 */
> >>-	*offset = ++toff;
> >>-	return nr_pages? ++nr_pages: 0;
> >>  }
> >
> >This section deletes code which is nice but there is a
> >problem. Your changelog says that this is duplicating the effort of
> >read_swap_cache_async() which is true but what it does is
> >
> >1. a swap cache lookup which will probably fail
> >2. alloc_page_vma()
> >3. radix_tree_preload()
> >4. swapcache_prepare
> >    - calls __swap_duplicate()
> >    - finds the hole, bails out
> >
> >That's a lot of work before the hole is found. Would it be worth
> >doing a racy check in swapin_readahead without swap lock held before
> >calling read_swap_cache_async()?
> 
> The problem is that without the swap_lock held, the swap_info
> struct may disappear completely because of the swapin_readahead
> happening concurrently with a swapoff.
> 

hmm, I considered that when I wrote the suggestion and was making
an assumption that the presense of swap pages on the area would
effectively pin the swap_info struct.

I did think of the race that swapoff is operating on the very last page
that swapin is reading and removes the map at the same time. We make a
racy check against effectively random memory and one of three results
might happen

1. DEBUG_PAGE_ALLOC was enabled and we blow up, ok, this one is bad if
   somewhat corner case

2. False positive count - we fall through and try to read the page,
   find the swap page no longer exists and bail gracefully

3. False zero count - we do not readin the swap page. It's not a big
   deal if swapin_readahead fails to read in a page it should have
   (swapoff has brought it in anyway).

Basically, I did not see any major downside with doing a racy check. If
1 above is a serious problem, then yes, we have to take the swap_lock
but that's still cheaper than allocating needlessly.

> I suspect that the CPU time spent doing 1-4 above will be
> negligible compared to the amount of time spent doing disk IO,

Negligible in comparison to the IO sure, but it still seems like we
could be doing a lot of allocation work (alloc_page_vma and
radix_tree_preload) for no good reason.

> but if there turns out to be a problem it should be possible
> to move the swap hole identification closer to the top of
> swap_cache_read_async().
> 

There is that, it'll be tricky to spot that we're needless allocating
pages though. Not the end of the world, what you have still improves
things.

-- 
Mel Gorman
SUSE Labs

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