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

Sergey Senozhatsky
2014-08-25T11:01:41Z

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.

> 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?

-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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>
> --
> Kind regards,
> Minchan Kim
>