Let's take the "find" command as an example. The command is defined here, and includes 33 fields at the time of writing this ticket. The generated parser uses a series of if / else if conditional statements to parse a BSON, by going through every element in the object:

void parseProtected(const IDLParserContext&, const OpMsgRequest& request) {
    ...
    for (const auto& element : request.body) {
        ...
        if (fieldName == kFilterFieldName) { ... }
        else if (fieldName == kProjectionFieldName) { ... }
        else if (fieldName == kSortFieldName) { ... }
        ...
    }
}

This will translate into multiple string comparisons for each element in the BSON object (on average 16 string comparisons). As a result, parsing commands shows up as a hot-spot in performance profiles, especially for low-latency workloads (e.g. YCSB 100% read). The performance implications scales with the number of fields in a command, and the order of if/else statements.

We can reduce the cost of parsing commands (both number of instructions and CPI) by either of the following:

• Reordering if/else statements based on the likelihood of taking each branch to reduce the total number of string comparisons.
• Avoid parsing the BSON object multiple times if possible (less pressure on the cache).
• Hashing filed names at compile-time, and using the hashed values to parse commands (my PoC).

Here are some performance numbers for my PoC using micro-benchmarks:

Running build/install/bin/find_command_bm
Run on (8 X 3501.93 MHz CPU s)
CPU Caches:
  L1 Data 48 KiB (x4)
  L1 Instruction 32 KiB (x4)
  L2 Unified 1280 KiB (x4)
  L3 Unified 55296 KiB (x1)
Load Average: 0.86, 0.77, 0.54
--------------------------------------------------------------------------------------
Benchmark                            Time             CPU   Iterations UserCounters...
--------------------------------------------------------------------------------------
BM_ParseFindCommand                510 ns          510 ns      1373844 items_per_second=1.96008M/s
BM_ParseHeavyFindCommand           566 ns          566 ns      1262140 items_per_second=1.76706M/s
BM_ParseFindCommandPoC             287 ns          287 ns      2440443 items_per_second=3.48677M/s
BM_ParseHeavyFindCommandPoC        232 ns          232 ns      3011081 items_per_second=4.30192M/s