Date: Wed, 14 Oct 2020 22:58:56 -0700 From: Enji Cooper <yaneurabeya@gmail.com> To: Alex Richardson <arichardson@FreeBSD.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-all <svn-src-all@freebsd.org>, svn-src-head@freebsd.org Subject: Re: svn commit: r366697 - head/usr.bin/xinstall Message-ID: <463B3544-BF7B-49FF-82EA-ECB5D8FBB17B@gmail.com> In-Reply-To: <202010141228.09ECSg0D023438@repo.freebsd.org> References: <202010141228.09ECSg0D023438@repo.freebsd.org>
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> On Oct 14, 2020, at 5:28 AM, Alex Richardson <arichardson@FreeBSD.org> = wrote: >=20 > Author: arichardson > Date: Wed Oct 14 12:28:41 2020 > New Revision: 366697 > URL: https://svnweb.freebsd.org/changeset/base/366697 >=20 > Log: > install(1): Avoid unncessary fstatfs() calls and use mmap() based on = size >=20 > According to git blame the trymmap() function was added in 1996 to = skip > mmap() calls for NFS file systems. However, nowadays mmap() should be > perfectly safe even on NFS. Importantly, onl ufs and cd9660 file = systems > were whitelisted so we don't use mmap() on ZFS. It also prevents the = use > of mmap() when bootstrapping from macOS/Linux since on those systems = the > trymmap() function was always returning zero due to the missing = MFSNAMELEN > define. >=20 > This change keeps the trymmap() function but changes it to check = whether > using mmap() can reduce the number of system calls that are required. > Using mmap() only reduces the number of system calls if we need = multiple read() > syscalls, i.e. if the file size is > MAXBSIZE. However, mmap() is = more expensive > than read() so this sets the threshold at 4 fewer syscalls. = Additionally, for > larger file size mmap() can significantly increase the number of page = faults, > so avoid it in that case. >=20 > It's unclear whether using mmap() is ever faster than a read with an = appropriate > buffer size, but this change at least removes two unnecessary system = calls > for every file that is installed. >=20 > Reviewed By: markj > Differential Revision: https://reviews.freebsd.org/D26041 * Has this change been tested out with source filesystems other = than UFS/ZFS? Not all filesystems support mmap(2). * trymmap(..) seems to be less about computing whether or not = the filesystem should use mmap(2) after this change, but how it should = use mmap(2). Seems like a misnamed function call now. Cheers, -Enji=
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