Date: Thu, 12 Apr 2001 19:42:42 +0200 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: Matt Dillon <dillon@earth.backplane.com> Cc: Rik van Riel <riel@conectiva.com.br>, David Xu <bsddiy@21cn.com>, freebsd-hackers@FreeBSD.ORG Subject: Re: vm balance Message-ID: <57992.987097362@critter> In-Reply-To: Your message of "Thu, 12 Apr 2001 10:30:41 PDT." <200104121730.f3CHUf419830@earth.backplane.com>
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In message <200104121730.f3CHUf419830@earth.backplane.com>, Matt Dillon writes: > >: >:On Tue, 10 Apr 2001, Matt Dillon wrote: >: >:> It's randomness that will kill performance. You know the old saying >:> about caches: They only work if you get cache hits, otherwise >:> they only slow things down. >: >:I wonder ... how does FreeBSD handle negative directory entries? >: >:That is, /bin/sh looks through the PATH to search for some executable >:(eg grep) and doesn't find it in the first 3 directories. >: >:Does the vfs cache handle this or does FreeBSD have to go down into >:the filesystem code every time? >: >:Rik > > The namei cache stores negative hits. /usr/src/sys/kern/vfs_cache.c > cache_lookup() - if ncp->nc_vp (the vnode) is NULL, the cache entry > represents a negative hit. cache_enter() - vp may be passed as NULL > to create a negative cache entry. ufs/ufs/ufs_lookup.c, calls to > cache_enter() enters positive or negative lookups as appropriate. > You should also know that negative entries, since they have no objects to "hang from" and consequently would clog up the name-cache, are limited by the sysctl: debug.ncnegfactor: 16 which means that max 1/16 of the name cache entries can be negative entries. You can monitor the number of negative entries with the sysctl debug.numneg: 305 the value of "16" was rather arbitrarily chosen and better defaults may exist. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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