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POHow should I interpret the output of the ghc heap profiler?
 

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  1. POHow should I interpret the output of the ghc heap profiler?
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    <p>I have a server process implemented in haskell that acts as a simple in-memory db. Client processes can connect then add and retrieve data. The service uses more memory than I would expect, and I'm attempting to work out why.</p> <p>The crudest metric I have is linux "top". When I start the process I see an "VIRT" image size of ~27MB. After running a client to insert 60,000 data items, I see an image size of ~124MB.</p> <p>Running the process to capture GC statistics (+RTS -S), I see initially</p> <pre><code>Alloc Copied Live GC GC TOT TOT Page Flts bytes bytes bytes user elap user elap 28296 8388 9172 0.00 0.00 0.00 0.32 0 0 (Gen: 1) </code></pre> <p>and on adding the 60k items I see the live bytes grow smoothly to</p> <pre><code> ... 532940 14964 63672180 0.00 0.00 23.50 31.95 0 0 (Gen: 0) 532316 7704 63668672 0.00 0.00 23.50 31.95 0 0 (Gen: 0) 530512 9648 63677028 0.00 0.00 23.50 31.95 0 0 (Gen: 0) 531936 10796 63686488 0.00 0.00 23.51 31.96 0 0 (Gen: 0) 423260 10047016 63680532 0.03 0.03 23.53 31.99 0 0 (Gen: 1) 531864 6996 63693396 0.00 0.00 23.55 32.01 0 0 (Gen: 0) 531852 9160 63703536 0.00 0.00 23.55 32.01 0 0 (Gen: 0) 531888 9572 63711876 0.00 0.00 23.55 32.01 0 0 (Gen: 0) 531928 9716 63720128 0.00 0.00 23.55 32.01 0 0 (Gen: 0) 531856 9640 63728052 0.00 0.00 23.55 32.02 0 0 (Gen: 0) 529632 9280 63735824 0.00 0.00 23.56 32.02 0 0 (Gen: 0) 527948 8304 63742524 0.00 0.00 23.56 32.02 0 0 (Gen: 0) 528248 7152 63749180 0.00 0.00 23.56 32.02 0 0 (Gen: 0) 528240 6384 63756176 0.00 0.00 23.56 32.02 0 0 (Gen: 0) 341100 10050336 63731152 0.03 0.03 23.58 32.35 0 0 (Gen: 1) 5080 10049728 63705868 0.03 0.03 23.61 32.70 0 0 (Gen: 1) </code></pre> <p>This appears to be telling me that the heap has ~63MB of live data. This could well be consistent with numbers from top, by the time you add on stack space, code space, GC overhead etc.</p> <p>So I attempted to use the heap profiler to work out what's making up this 63MB. The results are confusing. Running with "+RTS -h", and looking at the generated hp file, the last and largest snapshot has:</p> <pre><code>containers-0.3.0.0:Data.Map.Bin 1820400 bytestring-0.9.1.7:Data.ByteString.Internal.PS 1336160 main:KV.Store.Memory.KeyTree 831972 main:KV.Types.KF_1 750328 base:GHC.ForeignPtr.PlainPtr 534464 base:Data.Maybe.Just 494832 THUNK 587140 </code></pre> <p>All of the other numbers in the snapshot are much smaller than this. Adding these up gives the peak memory usage as ~6MB, as reflected in the chart output:</p> <p><img src="https://i.stack.imgur.com/8MRTX.png" alt="enter image description here"></p> <p>Why is this inconsistent with the live bytes as shown in the GC statistics? It's hard to see how my data structures could be requiring 63MB, and the profiler says they are not. Where is the memory going?</p> <p>Thanks for any tips or pointers on this.</p> <p>Tim</p>
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      1. POHow should I interpret the output of the ghc heap profiler?
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