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

Minchan Kim
2014-08-26T05:07:49Z

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?

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)

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
> > >
> > >
> > > --
> > > 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
> >
>
> --
> 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