Ubuntu Manpage: bup-memtest - test bup memory usage statistics

NAME

       bup-memtest - test bup memory usage statistics

SYNOPSIS

       bup memtest [options...]

DESCRIPTION

       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.

OPTIONS

       -n, --number=number
              set  the  number of objects to search for during each cycle (ie.  before printing a
              line of output)

       -c, --cycles=cycles
              set the number of cycles (ie.  the number of lines of output after the first).  The
              first  line  of  output  is  always  0  (ie.  the baseline before searching for any
              objects).

       --ignore-midx
              ignore any .midx files created by bup midx.  This  allows  you  to  compare  memory
              performance with and without using midx.

       --existing
              search for existing objects instead of searching for random nonexistent ones.  This
              can greatly affect memory usage and performance.  Note that most of the  time,  bup
              save spends 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.

EXAMPLES

              $ 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

DISCUSSION

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.

BUP

       Part of the bup(1) suite.

AUTHORS

       Avery Pennarun <apenwarr@gmail.com>.