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. > > > 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@xxxxxxxxxxx> > > > > > --- > > > > > 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@xxxxxxxxxxx> > > > > > +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@xxxxxxxxx> > > > > > 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@xxxxxxxxx. For more info on Linux MM, > > > > > see: http://www.linux-mm.org/ . > > > > > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > > > > > > > > -- > > > > Kind regards, > > > > Minchan Kim > > > > > > > > > > -- > > > To unsubscribe, send a message with 'unsubscribe linux-mm' in > > > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > > > see: http://www.linux-mm.org/ . > > > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > > > > -- > > Kind regards, > > Minchan Kim > > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . 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