From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: ARC-Seal: i=1; a=rsa-sha256; t=1524014802; cv=none; d=google.com; s=arc-20160816; b=HaXQPjYcB2cGcLXiWbGOSJBnpkfPZtgCPfhJnLL9TJ0NnwyPTv0ZUsEnaipWloSz6U GCE27d7zmSsF4nbLAac4fks0ICbYeQQBGjnRU+Bqa7YdDT1MmIKiohxSd0OVVXQvci0E jdYQCpw6q3CNcJgy4cQ7T7sLeueilhab/mbZpzTnMmrpFEe0FV0QHXqbKS3+9fZ+ZK9M h8ZeNWadiwFvrSL7Di625NXE37LsQCPmDFFLXqkjNND0KZIP305cU3/AgsOScJy6HhmR U7Wv1KapNw4jeG1yPk3F3kNHg2cm5VpjZBuVaONFx9+qPgxcHBqblgNL3AxI8WvRgWfG bnVQ== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=user-agent:in-reply-to:content-disposition:mime-version:references :message-id:subject:cc:to:from:date:sender:dkim-signature :arc-authentication-results; bh=GVdaAUrfNOPleNKE+f+r1r0MTncWe+ez762J56rpQX8=; b=L4QTtFJE8T37dFHM7rzMm5d4foB1ZTWjBY+oM5kpK1f4hQyMF6DrDxZOd4+3iOh0bb Safe2MBC4ACKNKxs+diL0bVg2uw63klycDQ1ZOCWqQaB8n7eLuPnPk4BC1mYq9xllYyK ECl4whXzGcsEuTO/M+MINHYZBYgX41KRDud4Oy7aeELa9N8HPKGAXNuv8bf4UbZivznU J8HIP+bZpkathaFdlkfDo8g09266sldjVjEvPOAvPNWNOjgY52EQv4iU+Uos8xwMuhKT CG6a7TNRwN2P2HAA3EXTayzJOB+gk9h5L73lk9iVfoM+uegecHwSN0a7JHNQGmr0fzXt +Gpg== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=qR3m6xT8; spf=pass (google.com: domain of minchan.kim@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=minchan.kim@gmail.com Authentication-Results: mx.google.com; dkim=pass header.i=@gmail.com header.s=20161025 header.b=qR3m6xT8; spf=pass (google.com: domain of minchan.kim@gmail.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=minchan.kim@gmail.com X-Google-Smtp-Source: AIpwx49iwjFobannL3JfSjIA9l0jYQVtlMjI2DIiu9yDN3XjOozjcwpUCr67Q5eOGxBCCRv7Ccpmvg== Sender: Minchan Kim Date: Wed, 18 Apr 2018 10:26:36 +0900 From: Minchan Kim To: Andrew Morton Cc: LKML , Sergey Senozhatsky , Randy Dunlap , Greg Kroah-Hartman , Sergey Senozhatsky Subject: Re: [PATCH v5 4/4] zram: introduce zram memory tracking Message-ID: <20180418012636.GA196478@rodete-desktop-imager.corp.google.com> References: <20180416090946.63057-1-minchan@kernel.org> <20180416090946.63057-5-minchan@kernel.org> <20180417145921.eac3d6379b5bade6c4f1a091@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180417145921.eac3d6379b5bade6c4f1a091@linux-foundation.org> User-Agent: Mutt/1.9.2 (2017-12-15) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1597872002743020578?= X-GMAIL-MSGID: =?utf-8?q?1598045345093101606?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: Hi Andrew, On Tue, Apr 17, 2018 at 02:59:21PM -0700, Andrew Morton wrote: > On Mon, 16 Apr 2018 18:09:46 +0900 Minchan Kim wrote: > > > zRam as swap is useful for small memory device. However, swap means > > those pages on zram are mostly cold pages due to VM's LRU algorithm. > > Especially, once init data for application are touched for launching, > > they tend to be not accessed any more and finally swapped out. > > zRAM can store such cold pages as compressed form but it's pointless > > to keep in memory. Better idea is app developers free them directly > > rather than remaining them on heap. > > > > This patch tell us last access time of each block of zram via > > "cat /sys/kernel/debug/zram/zram0/block_state". > > > > The output is as follows, > > 300 75.033841 .wh > > 301 63.806904 s.. > > 302 63.806919 ..h > > > > First column is zram's block index and 3rh one represents symbol > > (s: same page w: written page to backing store h: huge page) of the > > block state. Second column represents usec time unit of the block > > was last accessed. So above example means the 300th block is accessed > > at 75.033851 second and it was huge so it was written to the backing > > store. > > > > Admin can leverage this information to catch cold|incompressible pages > > of process with *pagemap* once part of heaps are swapped out. > > A few things.. > > - Terms like "Admin can" and "Admin could" are worrisome. How do we > know that admins *will* use this? How do we know that we aren't > adding a bunch of stuff which nobody will find to be (sufficiently) > useful? For example, is there some userspace tool to which you are > contributing which will be updated to use this feature? Actually, I used this feature two years ago to find memory hogger although the feature was very fast prototyping. It was very useful to reduce memory cost in embedded space. The reason I am trying to upstream the feature is I need the feature again. :) Yub, I have a userspace tool to use the feature although it was not compatible with this new version. It should be updated with new format. I will find a time to submit the tool. > > - block_state's second column is in microseconds since some > undocumented time. But how is userspace to know how much time has > elapsed since the access? ie, "current time". It's a sched_clock so it should be elapsed time since the system boot. I should have written it explictly. I will fix it. > > - Is the sched_clock() return value suitable for exporting to > userspace? Is it monotonic? Is it consistent across CPUs, across > CPU hotadd/remove, across suspend/resume, etc? Does it run all the > way up to 2^64 on all CPU types, or will some processors wrap it at > (say) 32 bits? etcetera. Documentation/timers/timekeeping.txt > points out that suspend/resume can mess it up and that the counter > can drift between cpus. Good point! I just referenced it from ftrace because I thought the goal is similiar "no need to be exact unless the drift is frequent but wanted to be fast" AFAIK, ftrace/printk is active user of the function so if the problem happens frequently, it might be serious. :)