Commun. ACM最新文献_第7页

Efficient maximum flow algorithms 高效的最大流量算法

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2628036

A. Goldberg, R. Tarjan

Though maximum flow algorithms have a long history, revolutionary progress is still being made.

尽管最大流量算法有着悠久的历史，但革命性的进步仍在不断发生。

引用次数: 115

Example-based learning in computer-aided STEM education 计算机辅助STEM教育中的实例学习

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2634273

Sumit Gulwani

Example-based reasoning techniques developed for programming languages also help automate repetitive tasks in education.

为编程语言开发的基于示例的推理技术也有助于自动化教育中的重复任务。

引用次数: 51

Getting consensus for data replication: technical perspective 为数据复制达成共识:技术角度

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2632794

P. Bernstein

云计算系统中的数据应该具有高可用性。换言之,无论你何时接入系统,你在系统中保存的数据都应随时可用。实现该功能的标准机制——数据复制——需要维护每个用户数据的多个副本。复制数据本身并不能解决问题,因为可用的数据副本可能是过期的。具体的解释如下,假设系统维护了文件 x 的三个副本:x1, x2,和 x3。假设 x1 和 x2 当前可用, 而 x3 不可用。如果程序更新 x,则系统会把新值写入 x1 和 x2,但由于 x3 不可用,所以无法更新 x3。这不会引发问题,因为系统仍然有两个正确的副本。接下来,假设 x1 和 x2 发生故障,然后 x3 得以恢复。现在,问题出现了。如果用户读取 x, 系统能提供的最佳结果是返回 x3 的值。但是该副本是过期的——它没有包含最近的更新值。针对这一问题,早期的解决方案为多数一致。处理 x 的写入操作时,系统会分配一个单调递增的时间戳,然后在 x 的多数副本中写入 x 的值和时间戳。处理 x 的读取操作时,系统会读取 x 的多数副本,然后返回具有最大时间戳的值。由于任意两个多数副本的交集至少会包含一个副本,所以每次读操作能够返回之前在该数据上写入的值。在上文的例子中,读取操作只读取了一个副本,并非三个副本中的多数。因此,可能无法返回 x 的最新状态。如果使用了多数一致, 系统就不得不等待第二个副本变为可用,这样才能读够两个副本。接下来,它会返回在读到的两个副本中更接近当前时间的副本,因为该副本的值必定是最新的。 Gifford 把多数的概念扩展为加权多数,并将其称之为仲裁。每个副本都会被赋予权重。读取操作必须访问达到读取仲裁的副本—— 副本集合的权重至少达到 R。而写操作则必须更新达到写入仲裁的副本——副本集合的权重至少达到 W。设定 R+W 必须超过所有副本的总权重 N 后,我们可以确保每次读取均能读到在最近一次写入操作中写入的值。例如,设系统保存了 x 的四个副本,即 x1-x4。我们可以把 x1 和 x2 的权重设为 1;若 x3 和 x4 位于更可靠的服务器中,则把 x3 和 x4 的权重设为 2。这样得到 N=6。如果读取比写入更频繁,那么我们可以设 R=3,W=4,这样读取执行的任务会比写入少。因为 R+W>N,所以每次读取均能读到之前在该数据上写入的值。按理说,每次写入操作都必须更新数据的所有可用副本。但是, 读者也必须读多个副本,这颇为麻烦,因为它增加了开销和延迟。如果写入操作经常执行很快,这会特别棘手,因为在这种情况下,即使每次读取操作没有读取仲裁, 也极有可能读到最新的副本。如果读到过期数据的概率足够低,那么让读者读取少于仲裁数量的副本或许是可取的。实际上,某些云应用也正是这么做的。直到最近,这种通过猜测实现的折中方法才有所改观。原来对于读到的数据的过期性或承受某些过期风险后可期望获得的延迟减少缺乏界定。后面的论文阐述了一项突破性进展,它抛开了猜测,用名为概率有界过期性的原理分析取而代之。给定 R、W 和 N,论文的作者提出了一个公式用于计算读到的数据不属于最近 K 次写入值之一的概率。为了计算读到的数据不是在最近Δ时间单位内写入的值的概率, 他们使用了基于时间参数的蒙特卡罗模拟,且这些参数可以在运行的系统中通过测量得出。不仅如此, 他们基于开源记录管理器实现了该机制,让这种分析变得切实可行, 使得用户可以依据自己的判断对过期性和延迟进行权衡。而原先的猜测工作现在也变成了人们可以精心策划的目标。

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引用次数: 3

Slow search 缓慢的搜索

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2633041

J. Teevan, Kevyn Collins-Thompson, Ryen W. White, S. Dumais

Seeking to enrich the search experience by allowing for extra time and alternate resources.

寻求通过允许额外的时间和替代资源来丰富搜索体验。

引用次数: 18

Openism, IPism, fundamentalism, and pragmatism 开放主义、IPism、原教旨主义和实用主义

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2632265

Moshe Y. Vardi

On that day, U.S. financial-services firm Lehman Brothers filed for bankruptcy protection. This bankruptcy filing, the largest in U.S. history, threatened to turn the economic recession of the 2007–2008 financial crisis into a full-scale economic depression. Only the drastic measures taken by the U.S. government averted that catastrophic possibility. But, in the minds of many people, this event shredded the dogma of capitalism as the ultimate and best economic system, just as the dissolution of the Soviet Union on December 26, 1991, shredded the dogma of communism as the ultimate and best economic system. Of course, just as communists once did, many of today's capitalists blame these failures not on the system itself , but rather on its being " impurely " or " improperly " applied. Yet, history shows that fundamentalist ideas rarely work over the long term, and we are likely destined to move toward the more pragmatic reality of state-regulated free markets. Mark Carney, the governor of the Bank of England, recently asserted, " unchecked market fundamentalism can devour the social capital essential for the long-term dynamism of capitalism itself. " Just as ownership of tangible property was the main battleground over which capitalism and communism fought in the 20 th century, ownership of intellectual property is fast becoming the battleground in the 21 st century, with today's economy being increasingly driven by large corporations dependent on these intangible assets. At one end, " IP capitalists " view intellectual property as no different than tangible property. IP capitalists chafe at the limited term of copyright and see nothing wrong with the activity of patent-assertion entities (also known as " patent trolls "), who enforce patent rights in an attempt to collect licensing fees, without manufacturing products or supplying services based upon the underlying patents. At the other end, " IP communists " object to copyright protection and software patents, and even argue that software should be free. It is regrettable, I believe, that the open access (OA) movement found itself in the IP communist camp. OA advocates unrestricted online access to peer-reviewed scholarly research. On the face of it, this idea is seductively attractive. Who can object to unrestricted access to research? Furthermore, after seeing the price of scholarly publications escalate in the 1990s, open access seemed like a perfect solution; no more escalating subscription fees. But just like any other intellectual property, online publishing has fixed …

当天，美国金融服务公司雷曼兄弟(Lehman Brothers)申请破产保护。这是美国历史上最大规模的破产申请，有可能将2007-2008年金融危机的经济衰退变成全面的经济萧条。只有美国政府采取的严厉措施才避免了这种灾难性的可能性。但是，在许多人看来，这一事件粉碎了资本主义作为最终和最好的经济制度的教条，就像1991年12月26日苏联解体，粉碎了共产主义作为最终和最好的经济制度的教条一样。当然，就像共产主义者曾经做过的那样，今天的许多资本家并不把这些失败归咎于制度本身，而是归咎于制度“不纯粹”或“应用不当”。然而，历史表明，原教旨主义思想很少能长期发挥作用，我们很可能注定要走向更务实的现实，即国家监管的自由市场。英国央行(Bank of England)行长马克•卡尼(Mark Carney)最近断言，“不受约束的市场原教旨主义会吞噬对资本主义本身的长期活力至关重要的社会资本。”正如有形财产的所有权是20世纪资本主义和共产主义斗争的主要战场一样，知识产权的所有权正在迅速成为21世纪的战场，今天的经济越来越多地由依赖这些无形资产的大公司驱动。一方面，“知识产权资本家”认为知识产权与有形财产无异。知识产权资本家对有限的版权期限感到恼火，并认为专利主张实体(也被称为“专利巨魔”)的行为没有任何问题，这些实体执行专利权是为了收取许可费，而不是基于基础专利生产产品或提供服务。另一方面，“知识产权共产主义者”反对版权保护和软件专利，甚至主张软件应该免费。我认为，开放获取(OA)运动陷入知识产权共产主义阵营是令人遗憾的。OA提倡不受限制地在线访问同行评审的学术研究。从表面上看，这个想法很诱人。谁能反对无限制地获取研究成果?此外，在看到学术出版物的价格在20世纪90年代上涨之后，开放获取似乎是一个完美的解决方案;不再有不断升级的订阅费用。但就像任何其他知识产权一样，在线出版已经修复了……

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引用次数: 5

Why the U.S. is not ready for mandatory CS education 为什么美国还没有做好CS强制教育的准备

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2632036

M. Guzdial

The Communications Web site, http://cacm.acm.org, features more than a dozen bloggers in the BLOG@CACM community. In each issue of Communications, we'll publish selected posts or excerpts.

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http://cacm.acm.org/blogs/blog-cacm

Mark Guzdial considers the consequences of requiring all schoolchildren to study computer science.

通讯网站http://cacm.acm.org上有十几位博客作者，他们都是BLOG@CACM社区的成员。在每期的通讯中，我们将发表精选的帖子或摘录。Guzdial考虑了要求所有学生学习计算机科学的后果。

引用次数: 8

Researching the robot revolution 研究机器人革命

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2633039

F. Levy, R. Murnane

Considering a program for cross-disciplinary research between computer scientists and economists studying the effects of computers on work.

考虑一个计算机科学家和经济学家之间的跨学科研究项目，研究计算机对工作的影响。

引用次数: 1

Fostering computational literacy in science classrooms 在科学课堂中培养计算机素养

Commun. ACM

Pub Date : 2014-08-01 DOI: 10.1145/2633031