On Mon, 2010-12-20 at 13:07 -0800, Fernando Lopez-Lezcano wrote: > I would like to bring to the attention of the list another current usage > of the tmpfs mounted on /dev/shm in Fedora packages: > > Jack (the Jack Audio Connection Kit, jackaudio.org) has been using the > file api (apologies if my wording is not absolutely correct in unix > terms) on the tmpfs filesystem that is mounted on /dev/shm for a very > long time (10 years?). "/tmp" is not useful to Jack because Jack's > internal communication pipes can't be stored in any disk based journaled > filesystem as the latencies involved in accessing them cause glitches in > the audio streams handled by Jack. This is right and wrong. JACK uses /dev/shm for two purposes on Linux [1]. The first is as the definition of what its configure script calls HOST_DEFAULT_TMP_DIR. This path is only used as a name to which to attach the jack sockets. The extent to which this will _ever_ touch the disk, even on a journaled filesystem, is: - eventually, the inode for that socket and the dnode for the containing directory will have to be written to the disk, once. - under memory pressure the vfs may decide to throw away the inode cache for that socket, which would then have to be re-read from disk for subsequent connecting JACK clients. In other words, these are setup costs, not maintenance costs. This may cause glitches in a realtime scenario to the extent that clients are created and destroyed, but in general I submit that the cost of exec() of those new clients is going to dwarf the cost of the inode cache miss for the JACK socket. [2] The other usage of /dev/shm is for actual shared memory segments, but the shm layer in jack uses shm_open() and friends, so the use of /dev/shm is simply glibc's implementation detail. [1] - I have read the JACK source literally once in my life (ie, just now), and I do not claim to be an expert, but this is all I was able to find. [2] - Though, should someone feel especially enterprising, it would probably be a worthwhile optimization to tweak the inode cache replacement to prefer dropping regular files to sockets, on the grounds that IPC should not be a disk operation. If it doesn't already; I haven't looked. - ajax -- devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxxx https://admin.fedoraproject.org/mailman/listinfo/devel
