From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.4 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C7F4C3F2CD for ; Fri, 28 Feb 2020 15:32:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4F8DD2469F for ; Fri, 28 Feb 2020 15:32:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="e4qoeDNu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726796AbgB1PcP (ORCPT ); Fri, 28 Feb 2020 10:32:15 -0500 Received: from us-smtp-delivery-1.mimecast.com ([205.139.110.120]:58392 "EHLO us-smtp-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726990AbgB1PcP (ORCPT ); Fri, 28 Feb 2020 10:32:15 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1582903934; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Bvw4EKIq7Nz96iliNwe+sYPmeB2MxKGQA5tmOw0Z4RU=; b=e4qoeDNugewOs2BzpMKD1l1eGuHpTjCRs8FQXjFMOIV3seKs1ErToZPmLrbsHO0DAsIKyG OKiFHGs2AnnuwsMRsM4zyKhmLH5NPDs5clDZG1JtJTRxO9x4WRP8Nrd+GvyT01oEpzM8uP EZpxo8kf4ui7Y7o9yLxD7zc8ssUHUeY= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-320-ajKGKpX8OVSU15sG2IeCPA-1; Fri, 28 Feb 2020 10:32:10 -0500 X-MC-Unique: ajKGKpX8OVSU15sG2IeCPA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id A0043107B7D8; Fri, 28 Feb 2020 15:32:08 +0000 (UTC) Received: from llong.remote.csb (ovpn-123-107.rdu2.redhat.com [10.10.123.107]) by smtp.corp.redhat.com (Postfix) with ESMTP id 72E28101D481; Fri, 28 Feb 2020 15:32:03 +0000 (UTC) Subject: Re: [PATCH 00/11] fs/dcache: Limit # of negative dentries To: Matthew Wilcox , Ian Kent Cc: Andreas Dilger , Alexander Viro , Jonathan Corbet , Luis Chamberlain , Kees Cook , Iurii Zaikin , Linux Kernel Mailing List , Linux FS Devel , linux-doc@vger.kernel.org, Mauro Carvalho Chehab , Eric Biggers , Dave Chinner , Eric Sandeen References: <20200226161404.14136-1-longman@redhat.com> <20200226162954.GC24185@bombadil.infradead.org> <2EDB6FFC-C649-4C80-999B-945678F5CE87@dilger.ca> <9d7b76c32d09492137a253e692624856388693db.camel@themaw.net> <20200228033412.GD29971@bombadil.infradead.org> From: Waiman Long Organization: Red Hat Message-ID: Date: Fri, 28 Feb 2020 10:32:02 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20200228033412.GD29971@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Sender: linux-doc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On 2/27/20 10:34 PM, Matthew Wilcox wrote: > On Thu, Feb 27, 2020 at 05:55:43PM +0800, Ian Kent wrote: >> Not all file systems even produce negative hashed dentries. >> >> The most beneficial use of them is to improve performance of rapid >> fire lookups for non-existent names. Longer lived negative hashed >> dentries don't give much benefit at all unless they suddenly have >> lots of hits and that would cost a single allocation on the first >> lookup if the dentry ttl expired and the dentry discarded. >> >> A ttl (say jiffies) set at appropriate times could be a better >> choice all round, no sysctl values at all. > The canonical argument in favour of negative dentries is to improve > application startup time as every application searches the library path > for the same libraries. Only they don't do that any more: > > $ strace -e file cat /dev/null > execve("/bin/cat", ["cat", "/dev/null"], 0x7ffd5f7ddda8 /* 44 vars */) = 0 > access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory) > openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3 > openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 > openat(AT_FDCWD, "/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3 > openat(AT_FDCWD, "/dev/null", O_RDONLY) = 3 > > So, are they still useful? Or should we, say, keep at most 100 around? > It is the shell that does the path search, not the command itself. Cheers, Longman