Re: [PATCH v3] zram: add num_discards for discarded pages stat

Sergey Senozhatsky
2014-08-26T14:16:21Z

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?

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@samsung.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@samsung.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@gmail.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
> > > >
> > > >
> > > > --
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> > > > see: http://www.linux-mm.org/ .
> > > > Don't email: email@kvack.org
> > >
> > > --
> > > Kind regards,
> > > Minchan Kim
> > >
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org. For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: email@kvack.org
>
> --
> Kind regards,
> Minchan Kim
>