bup-memtest - test bup memory usage statistics
bup memtest [options...]
bup memtest opens the list of pack indexes in your bup repository, then searches the list for a series of nonexistent objects, printing memory usage statistics after each cycle.
Because of the way Unix systems work, the output will usually show a large (and unchanging) value in the VmSize column, because mapping the index files in the first place takes a certain amount of virtual address space. However, this virtual memory usage is entirely virtual; it doesn't take any of your RAM. Over time, bup uses parts of the indexes, which need to be loaded from disk, and this is what causes an increase in the VmRSS column.
.midxfiles created by
bup midx. This allows you to compare memory performance with and without using midx.
bup savespends most of its time searching for nonexistent objects, since existing ones are probably in unmodified files that we won't be trying to back up anyway. So the default behaviour reflects real bup performance more accurately. But you might want this option anyway just to make sure you haven't made searching for existing objects much worse than before.
$ bup memtest -n300 -c5 PackIdxList: using 1 index. VmSize VmRSS VmData VmStk 0 20824 kB 4528 kB 1980 kB 84 kB 300 20828 kB 5828 kB 1984 kB 84 kB 600 20828 kB 6844 kB 1984 kB 84 kB 900 20828 kB 7836 kB 1984 kB 84 kB 1200 20828 kB 8736 kB 1984 kB 84 kB 1500 20828 kB 9452 kB 1984 kB 84 kB $ bup memtest -n300 -c5 --ignore-midx PackIdxList: using 361 indexes. VmSize VmRSS VmData VmStk 0 27444 kB 6552 kB 2516 kB 84 kB 300 27448 kB 15832 kB 2520 kB 84 kB 600 27448 kB 17220 kB 2520 kB 84 kB 900 27448 kB 18012 kB 2520 kB 84 kB 1200 27448 kB 18388 kB 2520 kB 84 kB 1500 27448 kB 18556 kB 2520 kB 84 kB
When optimizing bup indexing, the first goal is to keep the VmRSS reasonably low. However, it might eventually be necessary to swap in all the indexes, simply because you're searching for a lot of objects, and this will cause your RSS to grow as large as VmSize eventually.
The key word here is eventually. As long as VmRSS grows reasonably slowly, the amount of disk activity caused by accessing pack indexes is reasonably small. If it grows quickly, bup will probably spend most of its time swapping index data from disk instead of actually running your backup, so backups will run very slowly.
The purpose of
bup memtest is to give you an idea of how fast your memory usage is growing, and to help in optimizing bup for better memory use. If you have memory problems you might be asked to send the output of
bup memtest to help diagnose the problems.
Tip: try using
bup midx -a or
bup midx -f to see if it helps reduce your memory usage.
Trivia: index memory usage in bup (or git) is only really a problem when adding a large number of previously unseen objects. This is because for each object, we need to absolutely confirm that it isn't already in the database, which requires us to search through all the existing pack indexes to ensure that none of them contain the object in question. In the more obvious case of searching for objects that do exist, the objects being searched for are typically related in some way, which means they probably all exist in a small number of packfiles, so memory usage will be constrained to just those packfile indexes.
Since git users typically don't add a lot of files in a single run, git doesn't really need a program like
bup midx. bup, on the other hand, spends most of its time backing up files it hasn't seen before, so its memory usage patterns are different.
Part of the