From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f200.google.com (mail-pl1-f200.google.com [209.85.214.200]) by kanga.kvack.org (Postfix) with ESMTP id 91DA28E0001 for ; Fri, 28 Dec 2018 20:41:10 -0500 (EST) Received: by mail-pl1-f200.google.com with SMTP id o23so19459383pll.0 for ; Fri, 28 Dec 2018 17:41:10 -0800 (PST) Received: from out30-132.freemail.mail.aliyun.com (out30-132.freemail.mail.aliyun.com. [115.124.30.132]) by mx.google.com with ESMTPS id bg9si36805767plb.317.2018.12.28.17.41.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 28 Dec 2018 17:41:09 -0800 (PST) Subject: Re: [v3 PATCH 1/2] mm: swap: check if swap backing device is congested or not References: <1545428420-126557-1-git-send-email-yang.shi@linux.alibaba.com> <20181228164246.4867201125a2123c8f6a6f9c@linux-foundation.org> From: Yang Shi Message-ID: <453db4f7-cff9-c3ec-4b37-3669ab85016f@linux.alibaba.com> Date: Fri, 28 Dec 2018 17:41:01 -0800 MIME-Version: 1.0 In-Reply-To: <20181228164246.4867201125a2123c8f6a6f9c@linux-foundation.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: ying.huang@intel.com, tim.c.chen@intel.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 12/28/18 4:42 PM, Andrew Morton wrote: > On Sat, 22 Dec 2018 05:40:19 +0800 Yang Shi wrote: > >> Swap readahead would read in a few pages regardless if the underlying >> device is busy or not. It may incur long waiting time if the device is >> congested, and it may also exacerbate the congestion. >> >> Use inode_read_congested() to check if the underlying device is busy or >> not like what file page readahead does. Get inode from swap_info_struct. >> Although we can add inode information in swap_address_space >> (address_space->host), it may lead some unexpected side effect, i.e. >> it may break mapping_cap_account_dirty(). Using inode from >> swap_info_struct seems simple and good enough. >> >> Just does the check in vma_cluster_readahead() since >> swap_vma_readahead() is just used for non-rotational device which >> much less likely has congestion than traditional HDD. >> >> Although swap slots may be consecutive on swap partition, it still may be >> fragmented on swap file. This check would help to reduce excessive stall >> for such case. > Some words about the observed effects of the patch would be more than > appropriate! Yes, sure. Actually, this could reduce the latency long tail of do_swap_page() on a congested system. The test on my virtual machine with emulated HDD shows: Without swap congestion check: page_fault1_thr-1490  [023]   129.311706: funcgraph_entry:      # 57377.796 us |  do_swap_page();  page_fault1_thr-1490  [023]   129.369103: funcgraph_entry: 5.642 us   |  do_swap_page();  page_fault1_thr-1490  [023]   129.369119: funcgraph_entry:      # 1289.592 us |  do_swap_page();  page_fault1_thr-1490  [023]   129.370411: funcgraph_entry: 4.957 us   |  do_swap_page();  page_fault1_thr-1490  [023]   129.370419: funcgraph_entry: 1.940 us   |  do_swap_page();  page_fault1_thr-1490  [023]   129.378847: funcgraph_entry:      # 1411.385 us |  do_swap_page();  page_fault1_thr-1490  [023]   129.380262: funcgraph_entry: 3.916 us   |  do_swap_page();  page_fault1_thr-1490  [023]   129.380275: funcgraph_entry:      # 4287.751 us |  do_swap_page(); With swap congestion check:       runtest.py-1417  [020]   301.925911: funcgraph_entry:      # 9870.146 us |  do_swap_page();       runtest.py-1417  [020]   301.935785: funcgraph_entry: 9.802 us   |  do_swap_page();       runtest.py-1417  [020]   301.935799: funcgraph_entry: 3.551 us   |  do_swap_page();       runtest.py-1417  [020]   301.935806: funcgraph_entry: 2.142 us   |  do_swap_page();       runtest.py-1417  [020]   301.935853: funcgraph_entry: 6.938 us   |  do_swap_page();       runtest.py-1417  [020]   301.935864: funcgraph_entry: 3.765 us   |  do_swap_page();       runtest.py-1417  [020]   301.935871: funcgraph_entry: 3.600 us   |  do_swap_page();       runtest.py-1417  [020]   301.935878: funcgraph_entry: 7.202 us   |  do_swap_page(); The long tail latency (>1000us) is reduced significantly. BTW, do you need I resend the patch with the above information appended into the commit log? Thanks, Yang