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10. DHAT: a dynamic heap analysis tool

10. DHAT: a dynamic heap analysis tool

To use this tool, you must specify --tool=dhat on the Valgrind command line.

10.1. Overview

DHAT is a tool for examining how programs use their heap allocations.

It tracks the allocated blocks, and inspects every memory access to find which block, if any, it is to. It presents, on an allocation point basis, information about these blocks such as sizes, lifetimes, numbers of reads and writes, and read and write patterns.

Using this information it is possible to identify allocation points with the following characteristics:

  • potential process-lifetime leaks: blocks allocated by the point just accumulate, and are freed only at the end of the run.

  • excessive turnover: points which chew through a lot of heap, even if it is not held onto for very long

  • excessively transient: points which allocate very short lived blocks

  • useless or underused allocations: blocks which are allocated but not completely filled in, or are filled in but not subsequently read.

  • blocks with inefficient layout -- areas never accessed, or with hot fields scattered throughout the block.

As with the Massif heap profiler, DHAT measures program progress by counting instructions, and so presents all age/time related figures as instruction counts. This sounds a little odd at first, but it makes runs repeatable in a way which is not possible if CPU time is used.

10.2. Using DHAT

First off, as for normal Valgrind use, you probably want to compile with debugging info (the -g option). But by contrast with normal Valgrind use, you probably do want to turn optimisation on, since you should profile your program as it will be normally run.

Second, you need to run your program under DHAT to gather the profiling information. You might need to reduce the --num-callers value to get reasonably-sized output files, especially if you are profiling a large program; some trial and error might be needed to find a good value.

Finally, you need to use DHAT's viewer (in a web browser) to get a detailed presentation of that information.

10.2.1. Running DHAT

To run DHAT on a program prog, run:

valgrind --tool=dhat prog

The program will execute (slowly). Upon completion, summary statistics that look like this will be printed:

==11514== Total:     823,849,731 bytes in 3,929,133 blocks
==11514== At t-gmax: 133,485,082 bytes in 436,521 blocks
==11514== At t-end:  258,002 bytes in 2,129 blocks
==11514== Reads:     2,807,182,810 bytes
==11514== Writes:    1,149,617,086 bytes

The first line shows how many heap blocks and bytes were allocated over the entire execution.

The second line shows how many heap blocks and bytes were alive at t-gmax, i.e. the time when the heap size reached its global maximum (as measured in bytes).

The third line shows how many heap blocks and bytes were alive at t-end, i.e. the end of execution. In other words, how many blocks and bytes were not explicitly freed.

The fourth and fifth lines show how many bytes within heap blocks were read and written during the entire execution.

These lines are moderately interesting at best. More useful information can be seen with DHAT's viewer.

10.2.2. Output File

As well as printing summary information, DHAT also writes more detailed profiling information to a file. By default this file is named dhat.out.<pid> (where <pid> is the program's process ID), but its name can be changed with the --dhat-out-file option. This file is JSON, and intended to be viewed by DHAT's viewer, which is described in the next section.

The default .<pid> suffix on the output file name serves two purposes. Firstly, it means you don't have to rename old log files that you don't want to overwrite. Secondly, and more importantly, it allows correct profiling with the --trace-children=yes option of programs that spawn child processes.

The output file can be big, many megabytes for large applications built with full debugging information.

10.3. DHAT's Viewer

DHAT's viewer can be run in a web browser by loading the file dh_view.html. Use the "Load" button to choose a DHAT output file to view.

If loading takes a long time, it might be worth re-running DHAT with a smaller --num-callers value to reduce the stack depths, because this can significantly reduce the size of DHAT's output files.

10.3.1. The Output Header

The first part of the output shows the program command and process ID. For example:

Invocation {
  Command: /home/njn/moz/rust0/build/x86_64-unknown-linux-gnu/stage2/bin/rustc --crate-name tuple_stress src/main.rs
  PID:     18816
}

The second part of the output shows the t-gmax and t-end values again. For example:

Times {
  t-gmax: 8,138,210,673 instrs (86.92% of program duration)
  t-end:  9,362,544,994 instrs
}

10.3.2. The AP Tree

The third part of the output is the largest and most interesting part, showing the allocation point (AP) tree.

10.3.2.1. Structure

The following image shows a screenshot of part of an AP tree. The font is very small because this screenshot is intended to demonstrate the high-level structure of the tree rather than the details within the text.

Like any tree, it has a root node, leaf nodes, and non-leaf nodes. The structure of the tree is shown by the lines connecting nodes. Child nodes are beneath their parent and indented one level.

The sub-trees beneath a non-leaf node can be collapsed or expanded by clicking on the node. It is useful to collapse sub-trees that you aren't interested in.

Colours are meaningful, and are intended to ease tree navigation, but the information they represent is also present within the text. (This means that colour-blind users are not denied any information.)

Each leaf node is coloured green. Each non-leaf node is coloured blue and has a down arrow () next to it when its sub-tree is expanded. Each non-leaf node is coloured yellow and has a left arrow () next to it when its sub-tree is collapsed.

The shade of green, blue or yellow used for a node indicate its significance. Darker shades represent greater significance (in terms of bytes or blocks).

Note that the entire output is text, even the arrows and lines connecting nodes. This means you can copy and paste any part of the output easily into an email, bug report, etc.

10.3.2.2. The Root Node

The root node looks like this:

AP 1/1 (25 children) {
  Total:     1,355,253,987 bytes (100%, 67,454.81/Minstr) in 5,943,417 blocks (100%, 295.82/Minstr), avg size 228.03 bytes, avg lifetime 3,134,692,250.67 instrs (15.6% of program duration)
  At t-gmax: 423,930,307 bytes (100%) in 1,575,682 blocks (100%), avg size 269.05 bytes
  At t-end:  258,002 bytes (100%) in 2,129 blocks (100%), avg size 121.18 bytes
  Reads:     5,478,606,988 bytes (100%, 272,685.7/Minstr), 4.04/byte
  Writes:    2,040,294,800 bytes (100%, 101,551.22/Minstr), 1.51/byte
  Allocated at {
    #0: [root]
  }
}

The root node covers the entire execution. The information is a superset of the information shown when DHAT ran, adding details such as allocation rates, average block sizes, block lifetimes, and read and write ratios. The next example will explain these in more detail.

10.3.2.3. Interior Nodes

AP nodes further down the tree show information about a subset of allocations. For example:

AP 1.1/25 (2 children) {
  Total:     54,533,440 bytes (4.02%, 2,714.28/Minstr) in 458,839 blocks (7.72%, 22.84/Minstr), avg size 118.85 bytes, avg lifetime 1,127,259,403.64 instrs (5.61% of program duration)
  At t-gmax: 0 bytes (0%) in 0 blocks (0%), avg size 0 bytes
  At t-end:  0 bytes (0%) in 0 blocks (0%), avg size 0 bytes
  Reads:     15,993,012 bytes (0.29%, 796.02/Minstr), 0.29/byte
  Writes:    20,974,752 bytes (1.03%, 1,043.97/Minstr), 0.38/byte
  Allocated at {
    #1: 0x95CACC9: alloc (alloc.rs:72)
    #2: 0x95CACC9: alloc (alloc.rs:148)
    #3: 0x95CACC9: reserve_internal<syntax::tokenstream::TokenStream,alloc::alloc::Global> (raw_vec.rs:669)
    #4: 0x95CACC9: reserve<syntax::tokenstream::TokenStream,alloc::alloc::Global> (raw_vec.rs:492)
    #5: 0x95CACC9: reserve<syntax::tokenstream::TokenStream> (vec.rs:460)
    #6: 0x95CACC9: push<syntax::tokenstream::TokenStream> (vec.rs:989)
    #7: 0x95CACC9: parse_token_trees_until_close_delim (tokentrees.rs:27)
    #8: 0x95CACC9: syntax::parse::lexer::tokentrees::<impl syntax::parse::lexer::StringReader<'a>>::parse_token_tree (tokentrees.rs:81)
  }
}

The first line indicates the node's position in the tree. The 1.1 is a unique identifier for the node and also says that it is the first child node 1 (which is the root). The /25 says that it is one of 25 children, i.e. it has 24 siblings. The (2 children) says that this node node has two children of its own.

Allocations are aggregated by their allocation stack trace. The Allocated at section shows the allocation stack trace that is shared by all the blocks covered by this node.

The Total line shows that this node accounts for 4.02% of all bytes allocated during execution, and 7.72% of all blocks. These percentages are useful for comparing the significance of different nodes within a single profile; an AP that accounts for 10% of bytes allocated is likely to be more interesting than one that accounts for 2%.

The Total line also shows allocation rates, measured in bytes and blocks per million instructions. These rates are useful for comparing the significance of nodes across profiles made with different workloads.

Finally, the Total line shows the average size and lifetimes of these blocks.

The At t-gmax line says shows that no blocks from this AP were alive when the global heap peak occurred. In other words, these blocks do not contribute at all to the global heap peak.

The At t-end line shows that no blocks were from this AP were alive at shutdown. In other words, all those blocks were explicitly freed before termination.

The Reads and Writes lines show how many bytes were read within this AP's blocks, the fraction this represents of all heap reads, and the read rate. Finally, it shows the read ratio, which is the number of reads per byte. In this case the number is 0.29, which is quite low -- if no byte was read twice, then only 29% of the allocated bytes, which means that at least 71% of the bytes were never read! This suggests that the blocks are being underutilized and might be worth optimizing.

The Writes lines is similar to the Reads line. In this case, at most 38% of the bytes are ever written, and at least 62% of the bytes were never written.

The Reads and Writes measurements suggest that the blocks are being under-utilised and might be worth optimizing. Having said that, this kind of under-utilisation is common in data structures that grow, such as vectors and hash tables, and isn't always fixable.

10.3.2.4. Leaf Nodes

This is a leaf node:

AP 1.1.1.1/2 {
  Total:     31,460,928 bytes (2.32%, 1,565.9/Minstr) in 262,171 blocks (4.41%, 13.05/Minstr), avg size 120 bytes, avg lifetime 986,406,885.05 instrs (4.91% of program duration)
  Max:       16,779,136 bytes in 65,543 blocks, avg size 256 bytes
  At t-gmax: 0 bytes (0%) in 0 blocks (0%), avg size 0 bytes
  At t-end:  0 bytes (0%) in 0 blocks (0%), avg size 0 bytes
  Reads:     5,964,704 bytes (0.11%, 296.88/Minstr), 0.19/byte
  Writes:    10,487,200 bytes (0.51%, 521.98/Minstr), 0.33/byte
  Allocated at {
    ^1: 0x95CACC9: alloc (alloc.rs:72)
    ^2: 0x95CACC9: alloc (alloc.rs:148)
    ^3: 0x95CACC9: reserve_internal<syntax::tokenstream::TokenStream,alloc::alloc::Global> (raw_vec.rs:669)
    ^4: 0x95CACC9: reserve<syntax::tokenstream::TokenStream,alloc::alloc::Global> (raw_vec.rs:492)
    ^5: 0x95CACC9: reserve<syntax::tokenstream::TokenStream> (vec.rs:460)
    ^6: 0x95CACC9: push<syntax::tokenstream::TokenStream> (vec.rs:989)
    ^7: 0x95CACC9: parse_token_trees_until_close_delim (tokentrees.rs:27)
    ^8: 0x95CACC9: syntax::parse::lexer::tokentrees::<impl syntax::parse::lexer::StringReader<'a>>::parse_token_tree (tokentrees.rs:81)
    ^9: 0x95CAC39: parse_token_trees_until_close_delim (tokentrees.rs:26)
    ^10: 0x95CAC39: syntax::parse::lexer::tokentrees::<impl syntax::parse::lexer::StringReader<'a>>::parse_token_tree (tokentrees.rs:81)
    #11: 0x95CAC39: parse_token_trees_until_close_delim (tokentrees.rs:26)
    #12: 0x95CAC39: syntax::parse::lexer::tokentrees::<impl syntax::parse::lexer::StringReader<'a>>::parse_token_tree (tokentrees.rs:81)
  }
}

The 1.1.1.1/2 indicates that this node is a great-grandchild of the root; is the first grandchild of the node in the previous example; and has no children.

Leaf nodes contain an additional Max line, indicating the peak memory use for the blocks covered by this AP. (This peak may have occurred at a time other than t-gmax.) In this case, 31,460,298 bytes were allocated from this AP, but the maximum size alive at once was 16,779,136 bytes.

Stack frames that begin with a ^ rather than a # are copied from ancestor nodes. (In this example, the first 8 frames are identical to those from the node in the previous example.) These frames could be found by tracing back through ancestor nodes, but that can be annoying, which is why they are duplicated. This also means that each node makes complete sense on its own.

10.3.2.5. Access Counts

If all blocks covered by an AP node have the same size, an additional Accesses field will be present. It indicates how the reads and writes within these blocks were distributed. For example:

Total:     8,388,672 bytes (0.62%, 417.53/Minstr) in 262,146 blocks (4.41%, 13.05/Minstr), avg size 32 bytes, avg lifetime 16,726,078,401.51 instrs (83.25% of program duration)
At t-gmax: 8,388,672 bytes (1.98%) in 262,146 blocks (16.64%), avg size 32 bytes
At t-end:  0 bytes (0%) in 0 blocks (0%), avg size 0 bytes
Reads:     9,109,682 bytes (0.17%, 453.41/Minstr), 1.09/byte
Writes:    7,340,088 bytes (0.36%, 365.34/Minstr), 0.88/byte
Accesses: {
  [  0]  65547 7 8 4 65529 〃 〃 〃 16 〃 〃 〃 12 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 〃 65542 〃 〃 〃 - - - - 
}

Every block covered by this AP was 32 bytes. Within all of those blocks, byte 0 was accessed (read or written) 65,547 times, byte 1 was accessed 7 times, byte 2 was accessed 8 times, and so on.

The ditto symbol () means "same access count as the previous byte".

A dash (-) means "zero". (It is used instead of 0 because it makes unaccessed regions more easily identifiable.)

The infinity symbol (, not present in this example) means "exceeded the maximum tracked count".

Block layout can often be inferred from counts. For example, these blocks probably have four separate byte-sized fields, followed by a four-byte field, and so on.

Access counts can be useful for identifying data alignment holes or other layout inefficiencies.

10.3.2.6. Aggregate Nodes

The AP tree is very large and many nodes represent tiny numbers of blocks and bytes. Therefore, DHAT's viewer aggregates insignificant nodes like this:

AP 1.14.2/2 {
  Total:     5,175 blocks (0.09%, 0.26/Minstr)
  Allocated at {
    [5 insignificant]
  }
}

Much of the detail is stripped away, leaving only basic measurements, along with an indication of how many nodes were aggregated together (5 in this case).

10.3.3. The Output Footer

Below the AP tree is a line like this:

AP significance threshold: total >= 59,434.17 blocks (1%)

It shows the function used to determine if an AP node is significant. All nodes that don't satisfy this function are aggregated. It is occasionally useful if you don't understand why an AP node has been aggregated. The exact threshold depends on the sort metric (see below).

Finally, the bottom of the page shows a legend that explains some of the terms, abbreviations and symbols used in the output.

10.3.4. Sort Metrics

The order in which sub-trees are sorted can be changed via the "Sort metric" drop-down menu at the top of DHAT's viewer. Different sort metrics can be useful for finding different things. Some sort metrics also incorporate some filtering, so that only nodes meeting a particular criteria are shown.

Total (bytes)

The total number of bytes allocated during the execution. Highly useful for evaluating heap churn, though not quite as useful as "Total (blocks)".

Total (blocks)

The total number of blocks allocated during the execution. Highly useful for evaluating heap churn; reducing the number of calls to the allocator can significantly speed up a program. This is the default sort metric.

Total (blocks), tiny

Like "Total (blocks)", but shows only very small blocks. Moderately useful, because such blocks are often easy to avoid allocating.

Total (blocks), short-lived

Like "Total (blocks)", but shows only very short-lived blocks. Moderately useful, because such blocks are often easy to avoid allocating.

Total (bytes), zero reads or zero writes

Like "Total (bytes)", but shows only blocks that are never read or never written to (or both). Highly useful, because such blocks indicate poor use of memory and are often easy to avoid allocating. For example, sometimes a block is allocated and written to but then only read if a condition C is true; in that case, it may be possible to delay creating the block until condition C is true. Alternatively, sometimes blocks are created and never used; such blocks are trivial to remove.

Total (blocks), zero reads or zero writes

Like "Total (bytes), zero reads or zero writes" but for blocks. Highly useful.

Total (bytes), low-access

Like "Total (bytes)", but shows only blocks that have low numbers of reads or low numbers of writes (or both). Moderately useful, because such blocks indicate poor use of memory.

Total (blocks), low-access

Like "Total (bytes), low-access", but for blocks.

At t-gmax (bytes)

This shows the breakdown of memory at the point of peak heap memory usage. Highly useful for reducing peak memory usage.

At t-end (bytes)

This shows the breakdown of memory at program termination. Highly useful for identifying process-lifetime leaks.

Reads (bytes)

The number of bytes read within heap blocks. Occasionally useful.

Reads (bytes), high-access

Like "Reads (bytes)", but only shows blocks with high read ratios. Occasionally useful for identifying hot areas of memory.

Writes (bytes)

Like "Reads (bytes)", but for writes. Occasionally useful.

Writes (bytes), high-access

Like "Reads (bytes), high-access", but for writes. Occasionally useful.

The values within a node that represent the chosen sort metric are shown in bold, so they stand out.

Here is part of an AP node found with "Total (blocks), tiny", showing blocks with an average size of only 8.67 bytes:

Total:     3,407,848 bytes (0.25%, 169.62/Minstr) in 393,214 blocks (6.62%, 19.57/Minstr), avg size 8.67 bytes, avg lifetime 1,167,795,629.1 instrs (5.81% of program duration)

Here is part of an AP node found with "Total (blocks), short-lived", showing blocks with an average lifetime of only 181.75 instructions:

Total:     23,068,584 bytes (1.7%, 1,148.19/Minstr) in 262,143 blocks (4.41%, 13.05/Minstr), avg size 88 bytes, avg lifetime 181.75 instrs (0% of program duration)

Here is an example of an AP identified with "Total (blocks), zero reads or zero writes", showing blocks that are allocated but never touched:

Total:     7,339,920 bytes (0.54%, 365.33/Minstr) in 262,140 blocks (4.41%, 13.05/Minstr), avg size 28 bytes, avg lifetime 1,141,103,997.69 instrs (5.68% of program duration)
Max:       3,669,960 bytes in 131,070 blocks, avg size 28 bytes
At t-gmax: 3,336,400 bytes (0.79%) in 119,157 blocks (7.56%), avg size 28 bytes
At t-end:  0 bytes (0%) in 0 blocks (0%), avg size 0 bytes
Reads:     0 bytes (0%, 0/Minstr), 0/byte
Writes:    0 bytes (0%, 0/Minstr), 0/byte

All the blocks identified by these APs are good candidates for optimization.

10.4. Treatment of realloc

realloc is a tricky function and there are several different ways that DHAT could handle it.

Imagine a malloc(100) call followed by a realloc(200) call. This combination is considered to add two to the total block count, and 300 bytes to the total bytes count. (An alternative would be to only add one to the total block count, and 200 bytes to the total bytes count, as if a single malloc(200) call had occurred. While this would be defensible from a semantic point of view, it is silly from an operational point of view, because making two calls to allocator functions is more expensive than one call, and DHAT is a profiler that aims to help with runtime costs.)

Furthermore, the implicit copying of the 100 bytes is added to the reads and writes counts. Without this, the read and write counts would be under-measured and misleading.

However, DHAT only increases the current heap size by 100 bytes for this combination, and does not change the current block count. (As opposed to increasing the current heap size by 200 bytes and then decreasing it by 100 bytes.) As a result, it can only increase the global heap peak (if indeed, this results in a new peak) by 100 bytes.

Finally, the allocation point assigned to the block allocated by the malloc(100) call is retained once the block is reallocated. Which means that all 300 bytes are attributed to that allocation point, and no separate allocation point is created for the realloc(200) call. This may be surprising, but it has one large benefit.

Imagine some code that starts with an empty buffer, and then gradually adds data to that buffer from numerous different points in the code, reallocating the buffer each time it gets full. (E.g. code generation in a compiler might work this way.) With the described approach, the first heap block and all subsequent heap blocks are attributed to the same allocation point. While this is something of a lie -- the first allocation point isn't actually responsible for the other allocations -- it is arguably better than having the allocation points spread around, in a distribution that unpredictably depends on whenever the reallocation points were triggered.

10.5. DHAT Command-line Options

DHAT-specific command-line options are:

--dhat-out-file=<file>

Write the profile data to file rather than to the default output file, dhat.out.<pid>. The %p and %q format specifiers can be used to embed the process ID and/or the contents of an environment variable in the name, as is the case for the core option --log-file.

Note that stacks by default have 12 frames. This may be more than necessary, in which case the --num-callers flag can be used to reduce the number, which may make DHAT run slightly faster.



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