From patchwork Thu Dec 5 15:09:32 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: SeongJae Park X-Patchwork-Id: 11274923 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 156C513A4 for ; Thu, 5 Dec 2019 15:10:21 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DDC042464F for ; Thu, 5 Dec 2019 15:10:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="OW9Yn/Za" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729489AbfLEPKU (ORCPT ); Thu, 5 Dec 2019 10:10:20 -0500 Received: from smtp-fw-9102.amazon.com ([207.171.184.29]:3944 "EHLO smtp-fw-9102.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729187AbfLEPKU (ORCPT ); Thu, 5 Dec 2019 10:10:20 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1575558619; x=1607094619; h=from:to:cc:subject:date:message-id:in-reply-to: references:mime-version; bh=SrfasP8IAPqd8vrwXng/eZJeZ7Albzeeh72IDkV4jQI=; b=OW9Yn/ZaWJ//CBZeq8rGeDvcuJIk918UrrPch8wATbipy3qQVv6yuHor oQBLoPu9CAF6fms32pZB2riXH5OPO6W/fEWKnGS7USneM5hAWewSv1hpF wGxcfyel2/oFm7ytwq4hIe+4BD2cadgXIoBj2Jowd0JGtKm83bW0oPLHZ 4=; IronPort-SDR: 6VkidmQTW/ue9jNioio6TGxd5tu1XHQKcxD/T+VEDx8tDzmBagjTN63qVU/t5lYWXCLkKtZIph 1/OCyc1d8QPg== X-IronPort-AV: E=Sophos;i="5.69,281,1571702400"; d="scan'208";a="11821889" Received: from sea32-co-svc-lb4-vlan3.sea.corp.amazon.com (HELO email-inbound-relay-1e-a70de69e.us-east-1.amazon.com) ([10.47.23.38]) by smtp-border-fw-out-9102.sea19.amazon.com with ESMTP; 05 Dec 2019 15:10:02 +0000 Received: from EX13MTAUEA001.ant.amazon.com (iad55-ws-svc-p15-lb9-vlan2.iad.amazon.com [10.40.159.162]) by email-inbound-relay-1e-a70de69e.us-east-1.amazon.com (Postfix) with ESMTPS id 93E92A201D; Thu, 5 Dec 2019 15:10:00 +0000 (UTC) Received: from EX13D31EUA001.ant.amazon.com (10.43.165.15) by EX13MTAUEA001.ant.amazon.com (10.43.61.82) with Microsoft SMTP Server (TLS) id 15.0.1367.3; Thu, 5 Dec 2019 15:09:59 +0000 Received: from u886c93fd17d25d.ant.amazon.com (10.43.162.171) by EX13D31EUA001.ant.amazon.com (10.43.165.15) with Microsoft SMTP Server (TLS) id 15.0.1367.3; Thu, 5 Dec 2019 15:09:55 +0000 From: SeongJae Park To: , , CC: , , , , SeongJae Park Subject: [PATCH v2 1/1] xen/blkback: Aggressively shrink page pools if a memory pressure is detected Date: Thu, 5 Dec 2019 16:09:32 +0100 Message-ID: <20191205150932.3793-2-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 In-Reply-To: <20191205150932.3793-1-sjpark@amazon.com> References: <20191205150932.3793-1-sjpark@amazon.com> MIME-Version: 1.0 X-Originating-IP: [10.43.162.171] X-ClientProxiedBy: EX13D12UWA004.ant.amazon.com (10.43.160.168) To EX13D31EUA001.ant.amazon.com (10.43.165.15) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org From: SeongJae Park Each `blkif` has a free pages pool for the grant mapping. The size of the pool starts from zero and be increased on demand while processing the I/O requests. If current I/O requests handling is finished or 100 milliseconds has passed since last I/O requests handling, it checks and shrinks the pool to not exceed the size limit, `max_buffer_pages`. Therefore, `blkfront` running guests can cause a memory pressure in the `blkback` running guest by attaching a large number of block devices and inducing I/O. System administrators can avoid such problematic situations by limiting the maximum number of devices each guest can attach. However, finding the optimal limit is not so easy. Improper set of the limit can result in the memory pressure or a resource underutilization. This commit avoids such problematic situations by shrinking the pools aggressively (further the limit) for a while (users can set this duration via a module parameter) if a memory pressure is detected. Discussions =========== The `blkback`'s original shrinking mechanism returns only pages in the pool which are not currently be used by `blkback`. In other words, the pages that will be shrunk are not mapped with foreign pages. Because this commit is changing only the shrink limit but uses the shrinking mechanism as is, this commit does not introduce improper mappings related security issues. Once a memory pressure is detected, this commit keeps the aggressive shrinking limit for a user-specified time duration. The duration should be neither too long nor too short. If it is too long, free pages pool shrinking overhead can reduce the I/O performance. If it is too short, `blkback` will not free enough pages to reduce the memory pressure. This commit sets the value as `10 milliseconds` by default because it is a short time in terms of I/O while it is a long time in terms of memory operations. Also, as the original shrinking mechanism works for at least every 100 milliseconds, this could be a somewhat reasonable choice. I also tested other durations (refer to the below section for more details) and confirmed that 10 milliseconds is the one that works best. That said, the proper duration depends on actual configurations and workloads. That's why this commit is allowing users to set it as their optimal value via the module parameter. Memory Pressure Test ==================== To show how this commit fixes the above mentioned memory pressure situation well, I configured a test environment on a xen-running system. On the `blkfront` running guest instances, I attach a large number of network-backed volume devices and induce I/O to those. Meanwhile, I measure the number of pages that swapped in and out on the `blkback` running guest. The test ran twice, once for the `blkback` before this commit and once for that after this commit. As shown below, this commit has dramatically reduced the memory pressure: pswpin pswpout before 76,672 185,799 after 212 3,325 Optimal Aggressive Shrinking Duration ------------------------------------- To find a best aggressive shrinking duration, I repeated the test with three different durations (1ms, 10ms, and 100ms). The results are as below: duration pswpin pswpout 1 852 6,424 10 212 3,325 100 203 3,340 As expected, the numbers have further decreased by increasing the duration, but the reduction stopped from the `10ms`. Based on this results, I chose the default duration as 10ms. Performance Overhead Test ========================= This commit could incur I/O performance degradation under severe memory pressure because the aggressive shrinking will require more page allocations per I/O. To show the overhead, I artificially made an aggressive pages pool shrinking situation and measured the I/O performance of a `blkfront` running guest. For the artificial shrinking, I set the `blkback.max_buffer_pages` using the `/sys/module/xen_blkback/parameters/max_buffer_pages` file. We set the value to `1024` and `0`. The `1024` is the default value. Setting the value as `0` is same to a situation doing the aggressive shrinking always (worst-case). For the I/O performance measurement, I use a simple `dd` command. Default Performance ------------------- [dom0]# echo 1024 > /sys/module/xen_blkback/parameters/max_buffer_pages [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8827 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8781 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8737 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8702 s, 38.7 MB/s Worst-case Performance ---------------------- [dom0]# echo 0 > /sys/module/xen_blkback/parameters/max_buffer_pages [instance]$ for i in {1..5}; do dd if=/dev/zero of=file bs=4k count=$((256*512)); sync; done 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 11.7257 s, 45.8 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.878 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8746 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8786 s, 38.7 MB/s 131072+0 records in 131072+0 records out 536870912 bytes (537 MB) copied, 13.8749 s, 38.7 MB/s In short, even worst case aggressive shrinking makes no visible performance degradation. I think this is due to the slow speed of the I/O. In other words, the additional page allocation overhead is hidden under the much slower I/O latency. Nevertheless, pleaset note that this is just a very simple and minimal test. Signed-off-by: SeongJae Park --- drivers/block/xen-blkback/blkback.c | 35 +++++++++++++++++++++++++++-- 1 file changed, 33 insertions(+), 2 deletions(-) diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c index 3666afa639d1..72d068328ef1 100644 --- a/drivers/block/xen-blkback/blkback.c +++ b/drivers/block/xen-blkback/blkback.c @@ -135,6 +135,31 @@ module_param(log_stats, int, 0644); /* Number of free pages to remove on each call to gnttab_free_pages */ #define NUM_BATCH_FREE_PAGES 10 +/* + * Once a memory pressure is detected, keep aggressive shrinking of the free + * page pools for this time (milliseconds) + */ +static int xen_blkif_aggressive_shrinking_duration = 10; +module_param_named(aggressive_shrinking_duration, + xen_blkif_aggressive_shrinking_duration, int, 0644); +MODULE_PARM_DESC(aggressive_shrinking_duration, +"Duration in ms to do aggressive shrinking when a memory pressure is detected"); + +static unsigned long xen_blk_mem_pressure_end; + +static unsigned long blkif_shrink_count(struct shrinker *shrinker, + struct shrink_control *sc) +{ + xen_blk_mem_pressure_end = jiffies + + msecs_to_jiffies(xen_blkif_aggressive_shrinking_duration); + return 0; +} + +static struct shrinker blkif_shrinker = { + .count_objects = blkif_shrink_count, + .seeks = DEFAULT_SEEKS, +}; + static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt) { return xen_blkif_pgrant_timeout && @@ -656,8 +681,11 @@ int xen_blkif_schedule(void *arg) ring->next_lru = jiffies + msecs_to_jiffies(LRU_INTERVAL); } - /* Shrink if we have more than xen_blkif_max_buffer_pages */ - shrink_free_pagepool(ring, xen_blkif_max_buffer_pages); + /* Shrink the free pages pool if it is too large. */ + if (time_before(jiffies, xen_blk_mem_pressure_end)) + shrink_free_pagepool(ring, 0); + else + shrink_free_pagepool(ring, xen_blkif_max_buffer_pages); if (log_stats && time_after(jiffies, ring->st_print)) print_stats(ring); @@ -1500,6 +1528,9 @@ static int __init xen_blkif_init(void) if (rc) goto failed_init; + if (register_shrinker(&blkif_shrinker)) + pr_warn("shrinker registration failed\n"); + failed_init: return rc; }