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How to Profile Databend

go pprof tool​

go tool pprof http://localhost:8080/debug/pprof/profile?seconds=20

Fetching profile over HTTP from http://localhost:8080/debug/pprof/profile?seconds=20
Saved profile in /home/bohu/pprof/pprof.cpu.007.pb.gz
Type: cpu
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 5011, 100% of 5011 total
Dropped 248 nodes (cum <= 25)
Showing top 10 nodes out of 204
      flat  flat%   sum%        cum   cum%
      5011   100%   100%       5011   100%  backtrace::backtrace::libunwind::trace
         0     0%   100%        162  3.23%  <&alloc::vec::Vec<T,A> as core::iter::traits::collect::IntoIterator>::into_iter
         0     0%   100%         45   0.9%  <&mut I as core::iter::traits::iterator::Iterator>::next
         0     0%   100%         77  1.54%  <[A] as core::slice::cmp::SlicePartialEq<B>>::equal
         0     0%   100%         35   0.7%  <[u8; 8] as ahash::convert::Convert<u64>>::convert
         0     0%   100%        199  3.97%  <[u8] as ahash::convert::ReadFromSlice>::read_last_u64
         0     0%   100%         73  1.46%  <[u8] as ahash::convert::ReadFromSlice>::read_last_u64::as_array
         0     0%   100%        220  4.39%  <[u8] as ahash::convert::ReadFromSlice>::read_u64
         0     0%   100%        701 13.99%  <ahash::fallback_hash::AHasher as core::hash::Hasher>::write
         0     0%   100%         26  0.52%  <ahash::random_state::RandomState as core::hash::BuildHasher>::build_hash

Or

go tool pprof -http=0.0.0.0:8081 $HOME/pprof/pprof.cpu.007.pb.gz

Memory profiling​

databend-query and databend-meta can be built optionally with jemalloc, which provides various memory profiling features.

Currently, it does not work on Mac, with either intel or ARM.

Bring up with memory profiling enabled​

  • Build databend-query and databend-meta with memory-profiling feature enabled

    Under the project root path, e.g. ~/workspace/fuse-query/: cargo build --features memory-profiling

  • Fire up databend, using environment variable MALLOC_CONF to enable memory profiling.

    For example: MALLOC_CONF=prof:true ./target/debug/databend-query

Examine memory usage​

There are several typical use case to examine memory usage:

Dump a heap snapshot from a running databend-query or databend-meta process.​

NOTE: currently, periodical heap prof dump is NOT supported. A "snapshot" of the heap prof is dump instead.

  • Using jeprof 

    jeprof ./target/debug/databend-query http://localhost:8080/debug/mem

    The TCP port 8080 is defined in the config entry admin_api_address. This command loads profiling data from a running server then enters jeprof interactive tool, e.g.:

        Using local file ./target/debug/databend-query.
        Gathering CPU profile from http://localhost:8080/debug/mem/pprof/profile?seconds=30 for 30 seconds to ~/jeprof/databend-query.1650949265.localhost
        Be patient...
        Wrote profile to /home/zhaobr/jeprof/databend-query.1650949265.localhost
        Welcome to jeprof!  For help, type 'help'.

    Use top to list functions ordered by memory allocated.

        (jeprof) top
        Total: 16.2 MB
            10.2  62.7%  62.7%     10.2  62.7% ::alloc
             6.0  37.3% 100.0%      6.0  37.3% ::alloc_zeroed
             0.0   0.0% 100.0%     10.2  62.7% ::allocate
             0.0   0.0% 100.0%      0.5   3.3% ::call
             0.0   0.0% 100.0%      4.0  24.7% ::default
             0.0   0.0% 100.0%      1.2   7.2% ::deref
             0.0   0.0% 100.0%      1.2   7.2% ::deref::__stability (inline)
             0.0   0.0% 100.0%      1.2   7.2% ::deref::__static_ref_initialize (inline)
             0.0   0.0% 100.0%      0.5   3.1% ::from
             0.0   0.0% 100.0%      9.2  56.6% ::from_iter
        (jeprof)

    Other jeprof commands can be found with help.

  • Download raw profile data: curl 'http://localhost:8080/debug/mem/pprof/profile?seconds=0' > mem.prof. Then one could analyze the profiling data with other tool.

Generate memory allocation call graph.​

The most common use case is to find memory leak. This can be done by comparing two memory profile before and after an interval:

curl 'http://localhost:8080/debug/mem/pprof/profile?seconds=0' > a.prof
sleep 10
curl 'http://localhost:8080/debug/mem/pprof/profile?seconds=0' > b.prof

Generate a call graph in pdf illustrating memory allocation during this interval:

jeprof \
    --show_bytes \
    --nodecount=1024 \
    --nodefraction=0.001 \
    --edgefraction=0.001 \
    --maxdegree=64 \
    --pdf \
    ./target/debug/databend-meta \
    --base=a.prof \
    b.prof \
    > mem.pdf

The output pdf would be: