Date:	Fri, 5 Sep 2014 00:43:31 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Chao Yu <chao2.yu@...sung.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, ngupta@...are.org,
	'Jerome Marchand' <jmarchan@...hat.com>,
	'Andrew Morton' <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] zram: add num_discards for discarded pages stat

Hello,

On (09/04/14 11:25), Minchan Kim wrote:
> Hello Sergey,
> 
> First of all, Sorry for late response.
> 
> On Tue, Aug 26, 2014 at 11:15:43PM +0900, Sergey Senozhatsky wrote:
> > Hello,
> > 
> > On (08/26/14 14:08), Minchan Kim wrote:
> > > Hi,
> > > 
> > > On Mon, Aug 25, 2014 at 08:01:18PM +0900, Sergey Senozhatsky wrote:
> > > > Hello,
> > > > 
> > > > On (08/25/14 09:36), Minchan Kim wrote:
> > > > > Hello Chao,
> > > > > 
> > > > > On Fri, Aug 22, 2014 at 04:21:01PM +0800, Chao Yu wrote:
> > > > > > Since we have supported handling discard request in this commit
> > > > > > f4659d8e620d08bd1a84a8aec5d2f5294a242764 (zram: support REQ_DISCARD), zram got
> > > > > > one more chance to free unused memory whenever received discard request. But
> > > > > > without stating for discard request, there is no method for user to know whether
> > > > > > discard request has been handled by zram or how many blocks were discarded by
> > > > > > zram when user wants to know the effect of discard.
> > > > > 
> > > > > My concern is that how much we are able to know the effect of discard
> > > > > exactly with your patch.
> > > > > 
> > > > > The issue I can think of is zram-swap discard.
> > > > > Now, zram handles notification from VM to free duplicated copy between
> > > > > VM-owned memory and zRAM-owned's one so discarding for zram-swap might
> > > > > be pointless overhead but your stat indicates lots of free page discarded
> > > > > without real freeing 
> > > > 
> > > > this is why I've moved stats accounting to the place where actual
> > > > zs_free() happens. and, frankly, I still would like to see the number
> > > > of zs_free() calls, rather than the number of slot free notifications
> > > > and REQ_DISCARD (or separately), because they all end up calling
> > > > zs_free(). iow, despite the call path, from the user point of view
> > > > they are just zs_free() -- the number of pages that's been freed by
> > > > the 3rd party and we had have to deal with that.
> > > 
> > > My qeustion is that what user can do with the only real freeing count?
> > > Could you give me a concret example?
> > 
> > for !swap device case it's identicall to `num_discarded'.
> > for swap device case, it's a bit more complicated (less convenient) if
> > we actually can receive both slot free and delayed REQ_DISCARDs.
> > 
> > > It's a just number of real freeing count so if you were admin, what
> > > do you expect from that? That's what I'd like to see in changelog.
> > > 
> > > > 
> > > > > so that user might think "We should keep enable
> > > > > swap discard for zRAM because the stat indicates it's really good".
> > > > > 
> > > > > In summary, wouldn't it better to have two?
> > > > > 
> > > > > num_discards,
> > > > > num_failed_discards?
> > > > 
> > > > do we actully need this? the only value I can think of (perhaps I'm
> > > > missing something) is that we can make sure that we need to support
> > > > both slot free and REQ_DISCARDS, or we can leave only REQ_DISCARDS.
> > > > is there anything else?
> > > 
> > > The secnario I imagined with two stat is how REQ_DISCARDS is effective
> > > from swap layer. Normally, slot free logic is called in advance
> > > when the page is zapped or swap read happens to avoid duplicate copy,
> > > so discard request from swap space would be just overhead without
> > > any benefit so we might guide zram-swap user don't use "swap -d".
> > > Otherwise, as failed_discard ratio is low, it means it would be
> > > better to remove swap slot free logic because swap discard works well
> > > without slot free hint.(Although I don't think)
> > 
> > yes, so it looks like it is a developer's stat - to make some
> > observations and to come up with some decisions. do we really
> > want to put it into release?
> 
> Agree. I was too specific for my purpose and it couldn't be
> a compelling reason to make it export for general purpose.
> 
> Actually, discard req sent by swap for getting free cluster
> shouldn't be success(i,e num_discarded should be zero) because
> zram_slot_free_notify will always free the duplicated copy
> in advance so user don't have any gain with 'swapon -d'.
> 
> Now, I agree with you that we shouldn't add more stat without
> compelling reason so it would be better to rename notify_free
> with discarded and move it in zram_free_page like your patch.
> https://lkml.org/lkml/2014/8/21/294
> 
> I will ask to Andrew to revert Chao's patch and pick your patch
> after a few days unless Chao has another opinion.
> 

no problem.

I, probably, was not clear enough. one of my objections was that
it is really easy to add a new stat file, and surprisingly hard to
remove it later, even a temporary one. because it's almost impossible
to beat the "someone might use it" argument.
just a side note, my whole impresion is that some of the stats, that we
export, belongs to /proc/diskstats, /sys/block/zramX/stat and frieds.
I didn't check but I think that some handy tools like iostat/etc are
using these files. though I have no idea how often (if ever) people
want to track (or at least see) zram in iostat or similar tools.



please let me know if I have to resend the patch.

	-ss

> > 
> > 
> > I'm not strongly against and we can proceed with Chao's patch.
> > 
> > 	-ss
> > 
> > > My point is I'm not saying you're wrong but adding a new stat is easy
> > > and I need a compelling reason that how it can help users.
> > > 
> > > Thanks.
> > > 
> > > > 
> > > > 	-ss
> > > > 
> > > > > For it, we should modify zram_free_page has return value.
> > > > > What do other guys think?
> > > > > 
> > > > > > 
> > > > > > In this patch, we add num_discards to stat discarded pages, and export it to
> > > > > > sysfs for users.
> > > > > > 
> > > > > > * From v1
> > > > > >  * Update zram document to show num_discards in statistics list.
> > > > > > 
> > > > > > * From v2
> > > > > >  * Update description of this patch with clear goal.
> > > > > > 
> > > > > > Signed-off-by: Chao Yu <chao2.yu@...sung.com>
> > > > > > ---
> > > > > >  Documentation/ABI/testing/sysfs-block-zram | 10 ++++++++++
> > > > > >  Documentation/blockdev/zram.txt            |  1 +
> > > > > >  drivers/block/zram/zram_drv.c              |  3 +++
> > > > > >  drivers/block/zram/zram_drv.h              |  1 +
> > > > > >  4 files changed, 15 insertions(+)
> > > > > > 
> > > > > > diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > index 70ec992..fa8936e 100644
> > > > > > --- a/Documentation/ABI/testing/sysfs-block-zram
> > > > > > +++ b/Documentation/ABI/testing/sysfs-block-zram
> > > > > > @@ -57,6 +57,16 @@ Description:
> > > > > >  		The failed_writes file is read-only and specifies the number of
> > > > > >  		failed writes happened on this device.
> > > > > >  
> > > > > > +
> > > > > > +What:		/sys/block/zram<id>/num_discards
> > > > > > +Date:		August 2014
> > > > > > +Contact:	Chao Yu <chao2.yu@...sung.com>
> > > > > > +Description:
> > > > > > +		The num_discards file is read-only and specifies the number of
> > > > > > +		physical blocks which are discarded by this device. These blocks
> > > > > > +		are included in discard request which is sended by filesystem as
> > > > > > +		the blocks are no longer used.
> > > > > > +
> > > > > >  What:		/sys/block/zram<id>/max_comp_streams
> > > > > >  Date:		February 2014
> > > > > >  Contact:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> > > > > > diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
> > > > > > index 0595c3f..e50e18b 100644
> > > > > > --- a/Documentation/blockdev/zram.txt
> > > > > > +++ b/Documentation/blockdev/zram.txt
> > > > > > @@ -89,6 +89,7 @@ size of the disk when not in use so a huge zram is wasteful.
> > > > > >  		num_writes
> > > > > >  		failed_reads
> > > > > >  		failed_writes
> > > > > > +		num_discards
> > > > > >  		invalid_io
> > > > > >  		notify_free
> > > > > >  		zero_pages
> > > > > > diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> > > > > > index d00831c..904e7a5 100644
> > > > > > --- a/drivers/block/zram/zram_drv.c
> > > > > > +++ b/drivers/block/zram/zram_drv.c
> > > > > > @@ -606,6 +606,7 @@ static void zram_bio_discard(struct zram *zram, u32 index,
> > > > > >  		bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > > > > >  		zram_free_page(zram, index);
> > > > > >  		bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > > > > > +		atomic64_inc(&zram->stats.num_discards);
> > > > > >  		index++;
> > > > > >  		n -= PAGE_SIZE;
> > > > > >  	}
> > > > > > @@ -866,6 +867,7 @@ ZRAM_ATTR_RO(num_reads);
> > > > > >  ZRAM_ATTR_RO(num_writes);
> > > > > >  ZRAM_ATTR_RO(failed_reads);
> > > > > >  ZRAM_ATTR_RO(failed_writes);
> > > > > > +ZRAM_ATTR_RO(num_discards);
> > > > > >  ZRAM_ATTR_RO(invalid_io);
> > > > > >  ZRAM_ATTR_RO(notify_free);
> > > > > >  ZRAM_ATTR_RO(zero_pages);
> > > > > > @@ -879,6 +881,7 @@ static struct attribute *zram_disk_attrs[] = {
> > > > > >  	&dev_attr_num_writes.attr,
> > > > > >  	&dev_attr_failed_reads.attr,
> > > > > >  	&dev_attr_failed_writes.attr,
> > > > > > +	&dev_attr_num_discards.attr,
> > > > > >  	&dev_attr_invalid_io.attr,
> > > > > >  	&dev_attr_notify_free.attr,
> > > > > >  	&dev_attr_zero_pages.attr,
> > > > > > diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> > > > > > index e0f725c..2994aaf 100644
> > > > > > --- a/drivers/block/zram/zram_drv.h
> > > > > > +++ b/drivers/block/zram/zram_drv.h
> > > > > > @@ -86,6 +86,7 @@ struct zram_stats {
> > > > > >  	atomic64_t num_writes;	/* --do-- */
> > > > > >  	atomic64_t failed_reads;	/* can happen when memory is too low */
> > > > > >  	atomic64_t failed_writes;	/* can happen when memory is too low */
> > > > > > +	atomic64_t num_discards;	/* no. of discarded pages */
> > > > > >  	atomic64_t invalid_io;	/* non-page-aligned I/O requests */
> > > > > >  	atomic64_t notify_free;	/* no. of swap slot free notifications */
> > > > > >  	atomic64_t zero_pages;		/* no. of zero filled pages */
> > > > > > -- 
> > > > > > 2.0.1.474.g72c7794
> > > > > > 
> > > > > > 
> > > > > > --
> > > > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > > > the body to majordomo@...ck.org.  For more info on Linux MM,
> > > > > > see: http://www.linux-mm.org/ .
> > > > > > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> > > > > 
> > > > > -- 
> > > > > Kind regards,
> > > > > Minchan Kim
> > > > > 
> > > > 
> > > > --
> > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > > > the body to majordomo@...ck.org.  For more info on Linux MM,
> > > > see: http://www.linux-mm.org/ .
> > > > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> > > 
> > > -- 
> > > Kind regards,
> > > Minchan Kim
> > > 
> > 
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@...ck.org.  For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a>
> 
> -- 
> Kind regards,
> Minchan Kim
> 
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