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

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Date:   Wed, 1 Mar 2023 12:54:56 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv2 5/6] zsmalloc: extend compaction statistics

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
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.