On Tue, 2016-04-12 at 16:40 +0700, Duy Nguyen wrote: > On Tue, Apr 12, 2016 at 6:27 AM, David Turner < > dturner@xxxxxxxxxxxxxxxx> wrote: > > On Fri, 2016-04-08 at 18:16 -0400, David Turner wrote: > > > And SHM on Macs works a bit differently than on Linux in at least > > > two > > > irritating ways. > > > > > > So, uh, new version to come once I actually make it work on Mac. > > > Probably Monday. > > > > I was chatting with a friend about this and he mentioned that SHM > > does > > not really fit well into the Unix "everything is a file" model. It > > lives in a separate namespace, and still requires most of the file > > -like > > operations just with funny names and a separate namespace: > > shm_open, > > shm_unlink. This weirdness is something I noticed in my porting > > work: > > on OS X, a shm name can only be 32 bytes long, requiring weird > > hacks. > > And on OSX, fstat on a shm fd is rounded up to the page size (!). > > There may also be other portability issues that I have not yet > > discovered. > > > > Instead, my friend suggests that we should just use files. For > > instance, we could do $TMPDIR/$index_helper_pid/shm-index.$sha. > > > > (I'm proposing $TMPDIR because it's cleaned up on reboot so we > > don't > > need any manual intervention or complicated gc schemes) > > > > What do folks think of this? > > I avoided actual files for two reasons > > - disk error rate is higher than memory one, and we might need > trailing SHA-1 back This only matters if we ever *read* the mmap off disk. But that will rarely happen -- usually, everything will stay in memory. Also, we never worry about disk errors for other git objects (e.g. blobs, commits, or trees), so it seems silly to worry for the index. > - access is slow (unless cached, but we can't be sure) We could solve this (and the other problem) with mlock. > If we can keep index-helper stuff on tmpfs or similar, then it would > address both, but that's even more OS-specific than shm. If we have a > good abstraction layer then people can put stuff on $TMPDIR on Mac, > for example. But then it's not full POSIX file interface anymore... > And you forgot Windows which does not strictly follow UNIX design. As I understand it, Windows does support MAP_SHARED but not shm_open. So Windows-specific stuff has to be done for shm_open but not for files. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html