On 05/06/2013 08:41 PM, Jeff King wrote: > On Mon, May 06, 2013 at 02:03:40PM +0200, Michael Haggerty wrote: > [...] >> The loose refs cache is only used by the for_each_ref() functions, not >> for looking up individual references. Another approach would be to >> change the top-level for_each_ref() functions to re-stat() all of the >> loose references within the namespace that interests it, *then* verify >> that the packed-ref cache is not stale, *then* start the iteration. >> Then there would be no need to re-stat() files during the iteration. >> (This would mean that we have to prohibit a second reference iteration >> from being started while one is already in progress.) > > Hmm. Thinking on this more, I'm not sure that we need to stat the loose > references at all. We do not need to care if the loose refs are up to > date or not. Well, we might care, but the point here is not to pretend > that we have an up-to-date atomic view of the loose refs; it is only to > make sure that the fallback-to-packed behavior does not lie to us about > the existence or value of a ref. > > IOW, it is OK to come up with a value for ref X that was true at the > beginning of the program, even if it has been simultaneously updated. > Our program can operate as if it happened in the instant it started, > even though in real life it takes longer. But it is _not_ OK to miss the > existence of a ref, or to come up with a value that it did not hold at > some point during the program (e.g., it is not OK to return some cruft > we wrote into the packed-refs file when we packed it three weeks ago). This all sounds correct to me. Another potential problem caused by the non-atomicity of loose reference reading could be to confuse reachability tests if process 1 is reading loose references while process 2 is renaming a reference: 1. Process 1 looks for refs/heads/aaaaa and finds it missing. 2. Process 2 renames zzzzz -> aaaaa 3. Process 1 looks for refs/heads/zzzzz and finds it missing. Process 2 would think that any objects pointed to by aaaaa (formerly zzzzz) are unreachable. This would be unfortunate if it is doing an upload-pack and very bad if it is doing a gc. I wonder if this could be a problem in practice? (Gee, wouldn't it be nice to keep reflogs for deleted refs? :-) ) > That is a weaker guarantee, and I think we can provide it with: > > 1. Load all loose refs into cache for a particular enumeration. > > 2. Make sure the packed-refs cache is up-to-date (by checking its > stat() information and reloading if necessary). > > 3. Run the usual iteration over the loose/packed ref caches. This sounds reasonable, too, though I'll need some more time to digest your suggestion in detail. > [...] >> Of course, clearing (part of) the loose reference cache invalidates any >> pointers that other callers might have retained to refnames in the old >> version of the cache. I've never really investigated what callers might >> hold onto such pointers under the assumption that they will live to the >> end of the process. > > My proposal above gets rid of the need to invalidate the loose refs > cache, so we can ignore that complexity. The same concern applies to invalidating the packed-ref cache, which you still want to do. >> Given all of this trouble, there is an obvious question: why do we have >> a loose reference cache in the first place? I think there are a few >> reasons: >> >> 1. In case one git process has to iterate through the same part of the >> reference namespace more than once. (Does this frequently happen?) > > I'm not sure how often it happens. There are a few obvious candidates if > you "git grep 'for_each[a-z_]*ref'", but I'm not sure if the actual > performance impact is measurable (the cache should be warm after the > first run-through). > >> 2. Reading a bunch of loose references at the same time is more >> efficient than reading them one by one interleaved with other file >> reads. (I think this is a significant win.) > > Makes some sense, though I don't recall whether it was ever measured. I think I measured it once and found it a significant benefit, though I can't remember whether this was with a warm cache or only with a cold cache. In fact, I experimented with some other strategies for loose reference loading for performance reasons. Currently loose references are read into the cache lazily, one directory at a time. I experimented with changes in both directions: * Preloading the whole tree of loose references before starting an iteration. As I recall, this was a performance *win*. It was on my to-do list of things to pursue when I have some free time (ha, ha). I mostly wanted to check first that there are not callers who abort the iteration soon after starting it. For example, imagine a caller who tries to determine "are there any tags at all" by iterating over "refs/tags" with a callback that just returns 1; such a caller would suffer the cost of reading all of the loose references in "refs/tags". * Lazy loading loose references one reference at a time. The ideas was that this would allow the reference cache to be used for single-reference lookups. This change alone caused a significant performance loss, so it would have had to be combined with code for preloading directories or subtrees before a for_each_ref() iteration. Michael -- Michael Haggerty mhagger@xxxxxxxxxxxx http://softwareswirl.blogspot.com/ -- 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