Mihaela Mihaljevic
2eb3741848
Each location step accumulated its context nodes' axis results into a Set<Node> to drop nodes reachable from more than one context, hashing and copying every node. The wrapped TreeNode is reference-counted, so each copy is ARC traffic, and the profile showed the Set insert, hashing, and Node copies dominating evaluation. De-duplication is needed only when two distinct context nodes can reach the same node. Skip it on a single context and on the axes whose results are disjoint per context (child, attribute, namespace, self): those produce the identical node sequence the Set would, since every insertion succeeds, with no hashing. Keep the Set on the axes that genuinely overlap (descendant, ancestor, parent, the siblings, following/preceding). Predicates stay applied per context and the path result still passes through the final order-and-unique pass, so results are identical. Measured xpath ~13x to ~10x slower than libxml2 on the 20k-item generated corpus via a same-state A/B, ~28% faster, same result. XPathNodeSetDedupTests pins that a node shared by many contexts through ancestor/parent/descendant is still counted once, the disjoint axes return every distinct result, and an overlapping union still de-duplicates. The full XPath suite and the Apache Xalan XSLT gold-output corpus stay green.
5.6 KiB
Benchmarks: PureXML vs libxml2 (#139)
bash scripts/benchmark.sh [items] [iterations] generates a deterministic
corpus (never vendored), builds PureXML in release and the C driver against
the macOS SDK's libxml2, runs both over the same bytes with internal timing
(best-of-N, IO excluded), and prints the comparison. Both drivers also
cross-check the same XPath count, so every benchmark run doubles as a
cross-implementation correctness check.
Baseline (first measurement)
20000 items, ~4.2 MB, best of 5, Apple Silicon (Studio), release builds:
| operation | PureXML | libxml2 | ratio |
|---|---|---|---|
| parse | 0.49 s | 0.016 s | ~30x |
| serialize | 0.059 s | 0.012 s | ~5x |
| xpath | 0.42 s | 0.012 s | ~36x |
These are the honest starting numbers the #139 optimization passes burn down; the table updates as passes land, never silently.
Current record (size series + real corpora, best of 3)
| corpus | parse | serialize | xpath |
|---|---|---|---|
| generated 4.2 MB | 0.34 s (18.7x) | 0.064 s (5.5x) | 0.15 s (11.9x) |
| generated 42 MB | 3.49 s (18.7x) | 0.63 s (5.6x) | 1.57 s (11.7x) |
| generated 210 MB | 19.3 s (19.7x) | 3.19 s (5.6x) | 9.4 s (libxml2: refused) |
| NASA ADC 25 MB (real) | 1.73 s (24.5x) | 0.42 s (8.0x) | 0.76 s (19.4x) |
| SwissProt 115 MB (real) | 11.0 s (22.0x) | 1.77 s (5.5x) | 5.0 s (18.7x) |
Two findings only the large corpora could surface:
- PureXML's document-order sort was quadratic on flat fan-outs (42 MB: 27 s in XPath, ratio 190x). Fixed; and the first fix attempt was itself quadratic in a worse place (an eager sibling-table built per predicate evaluation: one hour at 42 MB), caught by sampling the live run. The cache now early-outs single-node sets and builds a parent's table only on its second lookup. XPath at 42 MB: 27.1 s -> 1.57 s.
- libxml2 refuses XPath evaluation past its XPATH_MAX_NODESET_LENGTH cap (10M nodes): the 210 MB corpus query returns NULL by design. PureXML completes it in 9.4 s. The protective counterpart for hostile input is tracked as #143.
Scaling: all three PureXML operations are linear in document size (ratios flat from 4 MB to 210 MB), and there is no 2 GB integer cliff (64-bit offsets throughout; libxml2's xmlReadMemory takes an int byte count).
Optimization log
| pass | change | parse | xpath | serialize |
|---|---|---|---|---|
| baseline | - | 0.494 s (30x) | 0.413 s (37x) | 0.061 s (5.5x) |
| 1 | allocation-free Reader.matches() | 0.414 s (26x) | unchanged | unchanged |
| 2 | byte-backed Reader (owned UTF-8 storage, pointer-decoded scalars, byte-level literal match/consume) | 0.375 s (23x) | unchanged | unchanged |
| 3 | bulk content runs (plain-ASCII character data scanned and consumed at the byte level, String built once per run) | 0.335 s (20x) | unchanged | unchanged |
| 4 | byte-level dispatch + name/attribute scanning (non-buffering peekByte, byte-mode skipSpace, all-ASCII takeASCIIName, attributeRunBytes); keeps the lookahead buffer empty so the existing byte fast paths engage |
0.217 s (13x) | unchanged | unchanged |
| 5 | sawAmpersand short-circuit (ampersand-free text returns verbatim, skipping the reference-decode/split/findings pass) and detecting the text-run-closing < via peekByte instead of buffering it (the buffered < had cascaded the next element's whole dispatch and name scan onto the Character path) |
0.157 s (9.5x) | unchanged | unchanged |
| 6 | byte-level markup dispatch (the single byte after < selects end-tag/PI/declaration/start-tag, replacing up to five literal string comparisons per element with one byte peek) |
0.147 s (9.0x) | unchanged | unchanged |
| 7 | sawAmpersand short-circuit for attribute values (the byte scanner notes any &; an ampersand-free value skips the reference-decode pass and its full re-scan); measured against same-state baseline, every run below every baseline run |
0.143 s (~4% vs baseline) | unchanged | unchanged |
| 8 | serialize escaping fast path (text/attribute/comment/PI return the value unchanged when no character needs escaping, instead of rebuilding it character by character; byte-level scan for the escapable bytes) | unchanged | unchanged | 0.032 s (2.7x, from 5.5x) |
| 9 | skip cross-context node-set de-duplication on a single context or a disjoint axis (child/attribute/namespace/self); the Set<Node> hashing and copying was pure overhead where no duplicate can arise, same output (same-state A/B, ~28%) |
unchanged | 0.097 s (10x, from 13x) | unchanged |
Profile findings (first sample, parse path)
Top of stack, release build: String.Iterator.next (the per-character pull closure chain), _stringCompareWithSmolCheck (Character equality in matches/peek), String._uncheckedFromUTF8 and _allASCII (small-string materialization), bridgeObjectRelease/swift_release (ARC on Character buffers), UnicodeScalarView.distance. The Reader's architecture: one closure call per scalar, [Character] lookahead, Character-typed classification. The structural fix is a byte-level (UTF-8) scanner with scalar classification tables; that is its own #139 pass, designed, not patched.
Known structure of the gap
- Parse and XPath dominate; serialize is closest.
- The scanner-vs-tree split needs the SAX stage, blocked on #142 (the public streaming surface is internal today).
- Suspects to profile, in order: Character/grapheme-based scanning versus byte scanning, per-scalar reader buffering (#135), ARC traffic in tree construction, XPath node-set materialization on every axis step.
Rules
- Optimize only what these numbers indict, one suspect per pass, with the before/after recorded here.
- The corpus generator changes only with a note here, since ratios are only comparable over identical corpora.