Change Buffer #1453
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Change Buffer #1453
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BenchmarksComparisonBenchmark execution time: 2026-01-20 20:09:29 Comparing candidate commit 5a6f809 in PR branch Found 0 performance improvements and 1 performance regressions! Performance is the same for 56 metrics, 2 unstable metrics. scenario:profile_add_sample2_frames_x1000
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BaselineOmitted due to size. |
Codecov Report❌ Patch coverage is Additional details and impacted files@@ Coverage Diff @@
## main #1453 +/- ##
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- Coverage 71.29% 70.93% -0.36%
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Files 416 418 +2
Lines 66872 67233 +361
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+ Hits 47676 47694 +18
- Misses 19196 19539 +343
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Artifact Size Benchmark Reportaarch64-alpine-linux-musl
aarch64-apple-darwin
aarch64-unknown-linux-gnu
libdatadog-x64-windows
libdatadog-x86-windows
x86_64-alpine-linux-musl
x86_64-apple-darwin
x86_64-unknown-linux-gnu
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ekump
reviewed
Jan 21, 2026
| trace_span_counts: HashMap<u128, usize>, | ||
| tracer_service: T, | ||
| tracer_language: T, | ||
| pid: f64, |
Contributor
Author
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Not in trace exporter's config 🤷
ekump
reviewed
Jan 21, 2026
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| impl From<u64> for OpCode { | ||
| fn from(val: u64) -> Self { | ||
| unsafe { std::mem::transmute(val) } |
Contributor
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This is UB if someone sends an invalid opcode. Could you use try_from instead?
Something like...
impl TryFrom<u64> for OpCode {
type Error = anyhow::Error;
fn try_from(val: u64) -> Result<Self, Self::Error> {
match val {
0 => Ok(OpCode::Create),
...
_ => Err(anyhow!("invalid opcode: {}", val))
}
}
}You'll need to update BufferedOperation::from_buf and flush_change_buffer to handle the Result, but both of those functions return Results anyway, so that shouldn't be a problem.
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I'd like to keep the transmute and just bounds-check it in the try_from. SGTY?
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Is there any reason to do bound-check + transmute rather than exhaust matching?
Contributor
Author
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The main reason is to avoid having all the OpCodes listed out in two places. Maybe I can macro that away? 🤔
ekump
reviewed
Jan 21, 2026
| use crate::span::{Span, SpanText}; | ||
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| #[derive(Clone, Copy)] | ||
| pub struct ChangeBuffer(*const u8); |
Contributor
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Instead of a raw pointer, can the buffer be &mut [u8]? If we can do that, I think you can move the unsafe code to the FFI layer / caller where it better belongs. The caller would need to do something like
let slice = unsafe { std::slice::from_raw_parts(ptr, len) };
let change_buffer = ChangeBuffer::new(slice); If we can do that, then we won't need the unsafe code in get_num_raw where every read has the potential for UB if we get the pointer math wrong. I think you'd also be able to get rid of the unsafe Send and Sync definitions.
Contributor
Author
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The problem is lifetimes. In napi.rs, the Buffer object we get from Node.js will have a lifetime as long as the function call. That said, the informal promise is that the Buffer will persist on the JS side forever, as good as 'static. Raw pointers allowed me to get around that.
I think I can take it a step further and convert that back to a slice with new ownership so that we can avoid all the unsafe. I'll give it a try.
ekump
reviewed
Jan 21, 2026
| .insert(T::from_static_str("_dd.rule_psr"), rule); | ||
| } | ||
| if let Some(rule) = trace.sampling_limit_decision { | ||
| span.metrics.insert(T::from_static_str("_dd.;_psr"), rule); |
hoolioh
reviewed
Jan 27, 2026
| use crate::span::{Span, SpanText}; | ||
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| #[derive(Clone, Copy)] | ||
| pub struct ChangeBuffer(*const u8); |
Contributor
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If you get into trouble with lifetimes using a slice from the managed side maybe you can add an length field. That way you can enclose all the unsafe operations to just the constructor and a handful of private helpers to turn it into a slice. Something like:
pub struct ChangeBuffer {
ptr: *mut u8,
len: usize,
}
impl ChangeBuffer {
pub unsafe fn from_raw_parts(ptr: *const u8, len: usize) -> Self {
Self { ptr: ptr as *mut u8, len }
}
fn as_slice(&self) -> &[u8] {
unsafe { std::slice::from_raw_parts(self.ptr, self.len) }
}
fn as_mut_slice(&mut self) -> &mut [u8] {
unsafe { std::slice::from_raw_parts_mut(self.ptr, self.len) }
}
pub fn read_u64(&self, offset: usize) -> Result<u64> {
let slice = self.as_slice();
let bytes = slice.get(offset..offset + 8)
.context(format!("read_u64 out of bounds: offset={}, len={}", offset, self.len))?;
let array: [u8; 8] = bytes.try_into()
.map_err(|_| anyhow!("failed to convert slice to [u8; 8]"))?;
Ok(u64::from_le_bytes(array))
}
pub fn write_u64(&mut self, offset: usize, value: u64) -> Result<()> {
let slice = self.as_mut_slice();
let target = slice.get_mut(offset..offset + 8)
.context(format!("write_u64 out of bounds: offset={}, len={}", offset, self.len))?;
target.copy_from_slice(&value.to_le_bytes());
Ok(())
}
pub fn clear_count(&mut self) -> Result<()> {
self.write_u64(0, 0)
.context("failed to clear buffer count")
}
| #[derive(Clone, Copy)] | ||
| pub struct ChangeBuffer(*const u8); | ||
| unsafe impl Send for ChangeBuffer {} | ||
| unsafe impl Sync for ChangeBuffer {} |
Contributor
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Do you really have to implement Sync? Is this required by NapiRS? In that case I think it will be good to document the constraints and point out that synchronization must be provided externally.
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Author
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IIRC napi.rs needed it, but for a reason that does not apply to our usage. I'll try again without.
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| impl From<u64> for OpCode { | ||
| fn from(val: u64) -> Self { | ||
| unsafe { std::mem::transmute(val) } |
Contributor
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Is there any reason to do bound-check + transmute rather than exhaust matching?
Contributor
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This is more of a general comment. Since trace-utils is too crowded and we talked about refactoring it. Would it make sense to place this implmentation in a new crate? I don't say that I has to happen now. Maybe in a later PR when we split trace-utils. |
hoolioh
reviewed
Jan 27, 2026
| } | ||
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| #[derive(Default)] | ||
| pub struct ChangeBufferState<T: SpanText + Clone> { |
Contributor
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If having a non-cryptogrhapic hasher is an option you can use FxHashMap from rustc-hash that way you can squeeze some performance from the lookups.
hoolioh
reviewed
Jan 27, 2026
| fn copy_in_chunk_tags(&self, span: &mut Span<T>) { | ||
| if let Some(trace) = self.traces.get(&span.trace_id) { | ||
| span.meta.reserve(trace.meta.len()); | ||
| span.meta.extend(trace.meta.clone()); |
Contributor
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I belive that clone will create a temproray hashmap so in order to save that extra allocation it will probably better to iterate over the hashmap just cloning the elements rather than cloning the whole thing.
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