Re: [PATCH] zram: introduce per-device debug_stat sysfs node

[Minchan Kim <minchan@kernel.org>]

On Fri, May 13, 2016 at 04:05:53PM +0900, Sergey Senozhatsky wrote:
> On (05/13/16 15:58), Sergey Senozhatsky wrote:
> > On (05/13/16 15:23), Minchan Kim wrote:
> > [..]
> > > @@ -737,12 +737,12 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> > >  		zcomp_strm_release(zram->comp, zstrm);
> > >  		zstrm = NULL;
> > >  
> > > -		atomic64_inc(&zram->stats.num_recompress);
> > > -
> > >  		handle = zs_malloc(meta->mem_pool, clen,
> > >  				GFP_NOIO | __GFP_HIGHMEM);
> > > -		if (handle)
> > > +		if (handle) {
> > > +			atomic64_inc(&zram->stats.num_recompress);
> > >  			goto compress_again;
> > > +		}
> > 
> > not like a real concern...
> > 
> > the main (and only) purpose of num_recompress is to match performance
> > slowdowns and failed fast write paths (when the first zs_malloc() fails).
> > this matching is depending on successful second zs_malloc(), but if it's
> > also unsuccessful we would only increase failed_writes; w/o increasing
> > the failed fast write counter, while we actually would have failed fast
> > write and extra zs_malloc() [unaccounted in this case]. yet it's probably
> > a bit unlikely to happen, but still. well, just saying.
> 
> here I assume that the biggest contributor to re-compress latency is
> enabled preemption after zcomp_strm_release() and this second zs_malloc().
> the compression itself of a PAGE_SIZE buffer should be fast enough. so IOW
> we would pass down the slow path, but would not account it.

biggest contributors are 1. direct reclaim by second zsmalloc call +
                         2. recompression overhead.

If zram start to support high comp ratio but slow speed algorithm like zlib
2 might be higher than 1 in the future so let's not ignore 2 overhead.

Although 2 is smaller, your patch just accounts only direct reclaim but my
suggestion can count both 1 and 2 so isn't it better?

I don't know why it's arguable here. :)

> 
> 	-ss
> 
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