Jeff King <p...@peff.net> wrote: > On Mon, Feb 06, 2017 at 08:48:20PM +0000, Eric Wong wrote: > > > I haven't hit insurmountable performance problems, even on > > low-end hardware; especially since I started storing blob ids in > > Xapian itself, avoiding the expensive tree lookup via git. > > The painful thing is traversing the object graph for clones and fetches. > Bitmaps help, but you still have to generate them.
Yep. "public-inbox-init" defaults to enabling bitmaps in the config for this reason. > > The main problem seems to be tree size. Deepening (2/2/36 vs > > 2/38) might be an option (I think Peff brought that up); but it > > might be easier to switch to YYYYMM refs (working like > > logrotate) and rely on Xapian to tie the entire thing together. > > Yes, the hashing is definitely one issue. Some numbers here: > > > http://public-inbox.org/git/20160805092805.w3nwv2l6jkbuw...@sigill.intra.peff.net/ > > If you have C commits on a tree with T entries, you have to do C*T hash > lookups for a flat tree (for each commit, you have to see "yup, already > saw that object"). Sharding that across H entries at the top level drops > the tree cost from T to H + T/H (actually, it's a bit worse because we > have to read the secondary tree, too). Sharding again (at H') gets you > H + H' + T/H/H'. > > Let's imagine you do one message per commit, so C=T. At 400K messages, > that's about 160 billion hash lookups flat. At H=256, it's about 700 > million. If you shard again with H'=256, it's 200 million. After that, > the additive terms start to dominate, and it's not worth going any > further (and also, we're ignoring the extra-tree cost to each level). Just to make sure I'm following, here; the entire formulas are: C * H + H' + (T / H / H') # 2/2/36 C * H + (T / H) # 2/38 (current) Right? > At that point you're better off to start having fewer commits. I know > that the schema you use does put useful information into the commit > message, but it's also redundant with what's in the messages themselves. > And it sounds like you push most of that out to Xapian anyway. Yeah, there's no benefit to Xapian users for having any info in the commit. However, keeping commit-per-message is still important to me to for better robustness from hardware and network failures. But yes, historical stuff could be squashed into a single commit (much like how linux.git started with v2.6.12-rc2 without history). Perhaps some folks will care about NNTP article numbering being non-chronological... > Imagine your repo had one commit with 400K historical messages, and then > grouped the new messages so that on average we got about 10 messages per > commit (this doesn't seem unrealistic for something that commits every > few minutes; the messages tend to be bunched in time; I ran some > numbers against a 10-minute mark in the earlier message). > > Then after another 100K messages, we'd have C=10,001 and T=500K. With > two levels of hashing at 256 each, that's ~5 million hash lookups to > walk the graph. And those numbers would be reasonable for a hosting site > like GitHub. > > I don't know what C is for the kernel repo, but I suspect with the right > tuning it could be made into large-but-reasonable. LKML probably has an upper bound of 30K messages per month; so it could hit 100K in less than 4 months. Worst case might be 360K messages a year 360000 * (256 + 256 + ((360000 + old) / 256 / 256)) That's still at least 180 million hash lookups after a year or so of real-time updates; right? (But probably closer to 240 million if there's 10 million old messages in there. Instead, I think I will add an option to support logrotate-style monthly heads (YYYYMM); keeping 2/38 and C == T: 30000 * (256 + (30000 / 256)) => 11 million 30000 * (256 + 256 + (30000 / 256 / 256)) => 15 million The monthly heads would each be discontiguous history-wise; so Xapian would become a requirement for users of this option for Message-ID lookups, but histories would still be readable with "git log" One good side-effect of using monthly heads is --single-branch clones may be used if someone lacks the bandwidth or space to do a full mirror. I'm not sure if the server-side (pack reuse, bitmaps) will benefit other aside from bandwidth reductions, though. A (far-fetched) option I've considered would be to store entire messages in the commit and have no trees or blobs at all. But that would require a significant rework, and would also make Xapian a hard requirement for even checking if a message is deleted or not.
