From: Johannes Weiner <hannes@cmpxchg.org>
To: Yosry Ahmed <yosryahmed@google.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>,
Nhat Pham <nphamcs@gmail.com>,
akpm@linux-foundation.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, minchan@kernel.org,
ngupta@vflare.org, sjenning@redhat.com, ddstreet@ieee.org,
vitaly.wool@konsulko.com
Subject: Re: [PATCH v6 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order
Date: Wed, 23 Nov 2022 11:30:45 -0500 [thread overview]
Message-ID: <Y35KtVaR/FKNAhKY@cmpxchg.org> (raw)
In-Reply-To: <CAJD7tkaWmusZ3V0Jh-zvaW3Ypt5Jn1GvzXryDjy58HSQWW74Gg@mail.gmail.com>
On Wed, Nov 23, 2022 at 12:11:24AM -0800, Yosry Ahmed wrote:
> On Wed, Nov 23, 2022 at 12:02 AM Yosry Ahmed <yosryahmed@google.com> wrote:
> > On Tue, Nov 22, 2022 at 7:50 PM Sergey Senozhatsky
> > > There are no accesses to swapped out pages yes, but zspage holds multiple
> > > objects, which are compressed swapped out pages in this particular case.
> > > For example, zspage in class size 176 (bytes) can hold 93 objects per-zspage,
> > > that is 93 compressed swapped out pages. Consider ZS_FULL zspages which
> > > is at the tail of the LRU list. Suppose that we page-faulted 20 times and
> > > read 20 objects from that zspage, IOW zspage has been in use 20 times very
> > > recently, while writeback still considers it to be "not-used" and will
> > > evict it.
> > >
> > > So if this works for you then I'm fine. But we probably, like you suggested,
> > > can document a couple of things here - namely why WRITE access to zspage
> > > counts as "zspage is in use" but READ access to the same zspage does not
> > > count as "zspage is in use".
> Nonetheless, the fact that we refaulted an object in a zspage does not
> necessarily mean that other objects on the same are hotter than
> objects in other zspages IIUC.
Yes.
On allocation, we know that there is at least one hot object in the
page. On refault, the connection between objects in a page is weak.
And it's weaker on zsmalloc than with other backends due to the many
size classes making temporal grouping less likely. So I think you're
quite right, Segey, that a per-class LRU would be more accurate.
It's no-LRU < zspage-LRU < class-LRU < object-LRU.
Like Yosry said, the plan is to implement an object-LRU next as part
of the generalized LRU for zsmalloc, zbud and z3fold.
For now, the zspage LRU is an improvement to no-LRU. Our production
experiments confirmed that.
next prev parent reply other threads:[~2022-11-23 16:30 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-11-19 0:15 [PATCH v6 0/6] Implement writeback for zsmalloc Nhat Pham
2022-11-19 0:15 ` [PATCH v6 1/6] zswap: fix writeback lock ordering " Nhat Pham
2022-11-22 1:43 ` Sergey Senozhatsky
2022-11-19 0:15 ` [PATCH v6 2/6] zpool: clean out dead code Nhat Pham
2022-11-22 1:46 ` Sergey Senozhatsky
2022-11-19 0:15 ` [PATCH v6 3/6] zsmalloc: Consolidate zs_pool's migrate_lock and size_class's locks Nhat Pham
2022-11-19 0:15 ` [PATCH v6 4/6] zsmalloc: Add a LRU to zs_pool to keep track of zspages in LRU order Nhat Pham
2022-11-19 16:38 ` Johannes Weiner
2022-11-19 17:34 ` Minchan Kim
2022-11-22 1:52 ` Sergey Senozhatsky
2022-11-22 17:42 ` Johannes Weiner
2022-11-23 3:50 ` Sergey Senozhatsky
2022-11-23 8:02 ` Yosry Ahmed
2022-11-23 8:11 ` Yosry Ahmed
2022-11-23 16:30 ` Johannes Weiner [this message]
2022-11-24 3:29 ` Sergey Senozhatsky
2022-11-24 3:21 ` Sergey Senozhatsky
2022-11-23 3:58 ` Sergey Senozhatsky
2022-11-19 0:15 ` [PATCH v6 5/6] zsmalloc: Add zpool_ops field to zs_pool to store evict handlers Nhat Pham
2022-11-19 16:39 ` Johannes Weiner
2022-11-22 1:11 ` Sergey Senozhatsky
2022-11-19 0:15 ` [PATCH v6 6/6] zsmalloc: Implement writeback mechanism for zsmalloc Nhat Pham
2022-11-19 16:45 ` Johannes Weiner
2022-11-19 17:35 ` Minchan Kim
2022-11-22 1:40 ` Sergey Senozhatsky
2022-11-22 2:00 ` Sergey Senozhatsky
2022-11-22 2:15 ` Sergey Senozhatsky
2022-11-22 3:12 ` Johannes Weiner
2022-11-22 3:42 ` Sergey Senozhatsky
2022-11-22 6:09 ` Johannes Weiner
2022-11-22 6:35 ` Sergey Senozhatsky
2022-11-22 7:10 ` Johannes Weiner
2022-11-22 7:19 ` Sergey Senozhatsky
2022-11-22 6:37 ` Sergey Senozhatsky
2022-11-23 16:30 ` Nhat Pham
2022-11-23 17:27 ` Johannes Weiner
2022-11-23 17:18 ` Johannes Weiner
2022-11-21 19:29 ` [PATCH v6 0/6] Implement writeback " Nhat Pham
2022-11-23 19:26 ` Nhat Pham
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