Re: [RFC][PATCH 07/10] zsmalloc: introduce auto-compact support

[Minchan Kim <minchan@kernel.org>]

On Sat, May 30, 2015 at 12:05:25AM +0900, Sergey Senozhatsky wrote:
> perform class compaction in zs_free(), if zs_free() has created
> a ZS_ALMOST_EMPTY page. this is the most trivial `policy'.

Finally, I got realized your intention.

Actually, I had a plan to add /sys/block/zram0/compact_threshold_ratio
which means to compact automatically when compr_data_size/mem_used_total
is below than the threshold but I didn't try because it could be done
by usertool.

Another reason I didn't try the approach is that it could scan all of
zs_objects repeatedly withtout any freeing zspage in some corner cases,
which could be big overhead we should prevent so we might add some
heuristic. as an example, we could delay a few compaction trial when
we found a few previous trials as all fails.
It's simple design of mm/compaction.c to prevent pointless overhead
but historically it made pains several times and required more
complicated logics but it's still painful.

Other thing I found recently is that it's not always win zsmalloc
for zram is not fragmented. The fragmented space could be used
for storing upcoming compressed objects although it is wasted space
at the moment but if we don't have any hole(ie, fragment space)
via frequent compaction, zsmalloc should allocate a new zspage
which could be allocated on movable pageblock by fallback of
nonmovable pageblock request on highly memory pressure system
so it accelerates fragment problem of the system memory.

So, I want to pass the policy to userspace.
If we found it's really trobule on userspace, then, we need more
thinking.

Thanks.

> 
> probably it would make zs_can_compact() to return an estimated number
> of pages that potentially will be free and trigger auto-compaction
> only when it's above some limit (e.g. at least 4 zs pages); or put it
> under config option.
> 
> this also tweaks __zs_compact() -- we can't do reschedule
> anymore, waiting for new pages in the current class. so we
> compact as much as we can and return immediately if compaction
> is not possible anymore.
> 
> auto-compaction is not a replacement of manual compaction.
> 
> compiled linux kernel with auto-compaction:
> 
> cat /sys/block/zram0/mm_stat
> 2339885056 1601034235 1624076288        0 1624076288    19961     1106
> 
> performing additional manual compaction:
> 
> echo 1 > /sys/block/zram0/compact
> cat /sys/block/zram0/mm_stat
> 2339885056 1601034235 1624051712        0 1624076288    19961     1114
> 
> manual compaction was able to migrate additional 8 objects. so
> auto-compaction is 'good enough'.
> 
> TEST
> 
> this test copies a 1.3G linux kernel tar to mounted zram disk,
> and extracts it.
> 
> w/auto-compaction:
> 
> cat /sys/block/zram0/mm_stat
>  1171456    26006    86016        0    86016    32781        0
> 
> time tar xf linux-3.10.tar.gz -C linux
> 
> real    0m16.970s
> user    0m15.247s
> sys     0m8.477s
> 
> du -sh linux
> 2.0G    linux
> 
> cat /sys/block/zram0/mm_stat
> 3547353088 2993384270 3011088384        0 3011088384    24310      108
> 
> =====================================================================
> 
> w/o auto compaction:
> 
> cat /sys/block/zram0/mm_stat
>  1171456    26000    81920        0    81920    32781        0
> 
> time tar xf linux-3.10.tar.gz -C linux
> 
> real    0m16.983s
> user    0m15.267s
> sys     0m8.417s
> 
> du -sh linux
> 2.0G    linux
> 
> cat /sys/block/zram0/mm_stat
> 3548917760 2993566924 3011317760        0 3011317760    23928        0
> 
> =====================================================================
> 
> iozone shows that auto-compacted code runs faster in several
> tests, which is hardly trustworthy. anyway.
> 
> iozone -t 3 -R -r 16K -s 60M -I +Z
> 
>        test           base       auto-compact (compacted 66123 objs)
>    Initial write   1603682.25          1645112.38
>          Rewrite   2502243.31          2256570.31
>             Read   7040860.00          7130575.00
>          Re-read   7036490.75          7066744.25
>     Reverse Read   6617115.25          6155395.50
>      Stride read   6705085.50          6350030.38
>      Random read   6668497.75          6350129.38
>   Mixed workload   5494030.38          5091669.62
>     Random write   2526834.44          2500977.81
>           Pwrite   1656874.00          1663796.94
>            Pread   3322818.91          3359683.44
>           Fwrite   4090124.25          4099773.88
>            Fread   10358916.25         10324409.75
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> ---
>  mm/zsmalloc.c | 25 +++++++++++++------------
>  1 file changed, 13 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index c2a640a..70bf481 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1515,34 +1515,28 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)
>  
>  		while ((dst_page = isolate_target_page(class))) {
>  			cc.d_page = dst_page;
> -			/*
> -			 * If there is no more space in dst_page, resched
> -			 * and see if anyone had allocated another zspage.
> -			 */
> +
>  			if (!migrate_zspage(pool, class, &cc))
> -				break;
> +				goto out;
>  
>  			putback_zspage(pool, class, dst_page);
>  		}
>  
> -		/* Stop if we couldn't find slot */
> -		if (dst_page == NULL)
> +		if (!dst_page)
>  			break;
> -
>  		putback_zspage(pool, class, dst_page);
>  		putback_zspage(pool, class, src_page);
> -		spin_unlock(&class->lock);
> -		cond_resched();
> -		spin_lock(&class->lock);
>  	}
>  
> +out:
> +	if (dst_page)
> +		putback_zspage(pool, class, dst_page);
>  	if (src_page)
>  		putback_zspage(pool, class, src_page);
>  
>  	spin_unlock(&class->lock);
>  }
>  
> -
>  unsigned long zs_get_total_pages(struct zs_pool *pool)
>  {
>  	return atomic_long_read(&pool->pages_allocated);
> @@ -1741,6 +1735,13 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
>  	unpin_tag(handle);
>  
>  	free_handle(pool, handle);
> +
> +	/*
> +	 * actual fullness might have changed, __zs_compact() checks
> +	 * if compaction makes sense
> +	 */
> +	if (fullness == ZS_ALMOST_EMPTY)
> +		__zs_compact(pool, class);
>  }
>  EXPORT_SYMBOL_GPL(zs_free);
>  
> -- 
> 2.4.2.337.gfae46aa
> 

-- 
Kind regards,
Minchan Kim

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