Re: [PATCHv2 5/6] zsmalloc: extend compaction statistics

[Minchan Kim <minchan@kernel.org>]

On Wed, Mar 01, 2023 at 12:54:56PM +0900, Sergey Senozhatsky wrote:
> On (23/02/28 14:20), Minchan Kim wrote:
> > On Sun, Feb 26, 2023 at 12:55:45PM +0900, Sergey Senozhatsky wrote:
> > > On (23/02/23 15:51), Minchan Kim wrote:
> > > > On Thu, Feb 23, 2023 at 12:04:50PM +0900, Sergey Senozhatsky wrote:
> > > > > Extend zsmalloc zs_pool_stats with a new member that
> > > > > holds the number of objects pool compaction moved
> > > > > between pool pages.
> > > > 
> > > > I totally understand this new stat would be very useful for your
> > > > development but not sure it's really useful for workload tune or
> > > > monitoring.
> > > > 
> > > > Unless we have strong usecase, I'd like to avoid new stat.
> > > 
> > > The way I see is that it *can* give some interesting additional data to
> > > periodical compaction (the one is not triggeed by the shrinker): if the
> > > number of moves objects is relatively high but the number of comapcted
> > > (feeed) pages is relatively low then the system has fragmentation in
> > > small size classes (that tend to have many objects per zspage but not
> > > too many pages per zspage) and in this case the interval between
> > > periodical compactions probably can be increased. What do you think?
> > 
> > In the case, how could we get only data triggered by periodical munual
> > compaction?
> 
> Something very simple like
> 
> 	read zram mm_stat
> 	trigger comapction
> 	read zram mm_stat
> 
> can work in most cases, I guess. There can be memory pressure
> and shrinkers can compact the pool concurrently, in which case
> mm_stat will include shrinker impact, but that's probably not
> a problem. If system is under memory pressure then user space

Agreed.

> in general does not have to do comapction, since the kernel will
> handle it.
> 
> Just an idea. It feels like "pages compacted" on its own tells very
> little, but I don't insist on exporting that new stat.

I don't mind adding the simple metric but I want to add metric if
we have real usecase with handful of comments how they uses it
in real world.

Thanks.