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Thu, 03 Apr 2025 09:38:21 -0700 (PDT) Date: Thu, 3 Apr 2025 12:38:19 -0400 From: Johannes Weiner To: Nhat Pham Cc: akpm@linux-foundation.org, yosry.ahmed@linux.dev, chengming.zhou@linux.dev, sj@kernel.org, linux-mm@kvack.org, kernel-team@meta.com, linux-kernel@vger.kernel.org, gourry@gourry.net, ying.huang@linux.alibaba.com, jonathan.cameron@huawei.com, dan.j.williams@intel.com, linux-cxl@vger.kernel.org, minchan@kernel.org, senozhatsky@chromium.org Subject: Re: [PATCH v2] zsmalloc: prefer the the original page's node for compressed data Message-ID: <20250403163819.GA368504@cmpxchg.org> References: <20250402204416.3435994-1-nphamcs@gmail.com> Precedence: bulk X-Mailing-List: linux-cxl@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250402204416.3435994-1-nphamcs@gmail.com> On Wed, Apr 02, 2025 at 01:44:16PM -0700, Nhat Pham wrote: > Currently, zsmalloc, zswap's and zram's backend memory allocator, does > not enforce any policy for the allocation of memory for the compressed > data, instead just adopting the memory policy of the task entering > reclaim, or the default policy (prefer local node) if no such policy is > specified. This can lead to several pathological behaviors in > multi-node NUMA systems: > > 1. Systems with CXL-based memory tiering can encounter the following > inversion with zswap/zram: the coldest pages demoted to the CXL tier > can return to the high tier when they are reclaimed to compressed > swap, creating memory pressure on the high tier. > > 2. Consider a direct reclaimer scanning nodes in order of allocation > preference. If it ventures into remote nodes, the memory it > compresses there should stay there. Trying to shift those contents > over to the reclaiming thread's preferred node further *increases* > its local pressure, and provoking more spills. The remote node is > also the most likely to refault this data again. This undesirable > behavior was pointed out by Johannes Weiner in [1]. > > 3. For zswap writeback, the zswap entries are organized in > node-specific LRUs, based on the node placement of the original > pages, allowing for targeted zswap writeback for specific nodes. > > However, the compressed data of a zswap entry can be placed on a > different node from the LRU it is placed on. This means that reclaim > targeted at one node might not free up memory used for zswap entries > in that node, but instead reclaiming memory in a different node. > > All of these issues will be resolved if the compressed data go to the > same node as the original page. This patch encourages this behavior by > having zswap and zram pass the node of the original page to zsmalloc, > and have zsmalloc prefer the specified node if we need to allocate new > (zs)pages for the compressed data. > > Note that we are not strictly binding the allocation to the preferred > node. We still allow the allocation to fall back to other nodes when > the preferred node is full, or if we have zspages with slots available > on a different node. This is OK, and still a strict improvement over > the status quo: > > 1. On a system with demotion enabled, we will generally prefer > demotions over compressed swapping, and only swap when pages have > already gone to the lowest tier. This patch should achieve the > desired effect for the most part. > > 2. If the preferred node is out of memory, letting the compressed data > going to other nodes can be better than the alternative (OOMs, > keeping cold memory unreclaimed, disk swapping, etc.). > > 3. If the allocation go to a separate node because we have a zspage > with slots available, at least we're not creating extra immediate > memory pressure (since the space is already allocated). > > 3. While there can be mixings, we generally reclaim pages in > same-node batches, which encourage zspage grouping that is more > likely to go to the right node. > > 4. A strict binding would require partitioning zsmalloc by node, which > is more complicated, and more prone to regression, since it reduces > the storage density of zsmalloc. We need to evaluate the tradeoff > and benchmark carefully before adopting such an involved solution. > > [1]: https://lore.kernel.org/linux-mm/20250331165306.GC2110528@cmpxchg.org/ > > Suggested-by: Gregory Price > Signed-off-by: Nhat Pham Acked-by: Johannes Weiner