Bitcoin Core tests: performance in VM

Being able to run tests quickly is essential during development. As a maintainer I end up running the test suite several dozens of times per day. Recently I noticed that the functional tests (test/functional/ are slow when run inside a qemu VM (with KVM).

When I drilled down to find the cause it became apparent that RPC calls such as getnewaddress were much slower (up to 10 times, when compared to bare metal on older hardware). It turns out that this is because the wallet code does an fsync after every operation to make sure that changes to the database are safely wrtten to disk (for example in the case of a power failure), and these happen to be slow in this environment.

In tests, all state is deleted afterwards so this extra robustness isn’t useful. To see if it could be a faster I performed timings running the tests with various more and less safe cache settings.

qemu cache setting

The cache setting determines the caching strategy used for a virtual block device, and can be provided on the qemu command line though cache= or, in the case of libvirt is configured in the XML description, e.g.

<driver name='qemu' type='qcow2' cache='writeback'/>

tmp on qcow2 (image)

cache cumulative (s) runtime (s)
none 4138 1060
writethrough 4081 1044
default 3912 1003
writeback 3851 988
unsafe 1042 278

tmp on raw (block device)

cache cumulative (s) runtime (s)
none 3850 986
writethrough 3806 977
default 3300 840
writeback 3247 826
unsafe 987 264


Both in the case of a qcow2 virtual disk and when passing though a block device, the cache setting makes a significant impact on performance. unsafe caching, which disables fsync completely, results in the fastest test runs. This can make almost a factor 4 difference compared to the slowest option, none.

Besides changing the cache setting, a similar effect can be achieved using the utility eatmydata. This tool wraps a command and disables fsync and similar library calls completely while it runs. This results in comparable timings to using unsafe caching, and may also help on bare metal.

Travis CI, although it runs the tests in VMs, doesn’t seem to be affected by this: disabling fsync did not show influence on performance there. It’s possible that they’ve already configured unsafe caching.

Note: all of these suggestions compromise integrity for performance. Doing this when live wallets are involved would be an extremely bad idea.

Written on April 19, 2017
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