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Re: multiple sequential lmdb readers + spinning media = slow / thrashes?


If you are talking about rotational media, the more reader you add the
worse your aggregate bandwidth is going to be... Since LMDB is storing
it as a btree, the readers have to random access which turns into a
lot of seek. Seek time ends up being amortized as a higher average
time to read a block / page and your aggregate bandwidth disappears.

If you have enough memory to store most of the data, or your working
set it only a small subset of that data this won't be as visible.

- Milosz

On Thu, Feb 26, 2015 at 5:50 PM, Matthew Moskewicz
<moskewcz@alumni.princeton.edu> wrote:
> warnings: new to list, first post, lmdb noob.
> i'm a caffe user:
> https://github.com/BVLC/caffe
> in one use case, caffe sequentially streams though >100GB lmdbs at a rate of
> ~30MB/s in blocks of about 40MB. however, if multiple caffe processes are
> reading the same lmdb (opened with MDB_RDONLY), read performance becomes
> limiting (i.e. the processes become IO bound), even though the disk has
> sufficient read bandwidth (say ~180MB/s). some of the relevant caffe lmdb
> code is here:
> https://github.com/BVLC/caffe/blob/master/src/caffe/util/db.cpp
> however, if i *both*
> 1) run  blockdev --setra 65536 --setfra 65536 /dev/sdwhatever
> 2) modify lmdb to call posix_madvise(env->me_map, env->me_mapsize,
> then i can get >1 reader to run without being IO limited.
> for (2), see https://github.com/moskewcz/scratch/tree/lmdb_seq_read_opt
> similarly, using a sequential read microbenchmark designed to model the
> caffe reads from here:
> https://github.com/moskewcz/boda/blob/master/src/lmdbif.cc
> if i run one reader, i get 180MB/s bandwidth.
> with two readers, but neither (1) nor (2) above, each gets ~30MB/s
> bandwidth.
> with (1) and (2) enabled, and two readers, each gets ~90MB/s bandwidth.
> any advice?
> mwm
> PS: backstory (skippable):
> caffe originally used LevelDB to get better read performance for
> sequentially loading sets of ~1M 227x227x3 raw images (~200GB data).
> typically processing time is ~2 hours for this data set size, yielding a
> read BW need of 30MB/s or so. it's not really clear if/why LevelDB was uses
> aside from the fact that the caffe author was a google intern at the time he
> wrote it, but anecdotally i think the claim is that reading the raw .jpgs
> had perf. issues, although it's unclear exactly what or why. i guess it was
> the usual story about not getting sequential reads without using LevelDB.
> they switched to lmdb a while back.

Milosz Tanski
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