datasette/perf/request-response

Datasette request/response performance on Python 3.15

This directory holds a profiling study of Datasette's core URL handling and request/response cycle at commit cf113a5. It covers routing, middleware, Request/Response, cookies and signing, URL building, template plumbing, static files and error paths. Optimising SQL queries against user data was out of scope. Per-request work that only touches SQLite without being a data query, such as probing a throwaway :memory: connection on every request, was in scope and is labelled "DB-adjacent".

Everything was measured on Python 3.15.0b4 (installed with uv python install 3.15). Profiling used the new statistical sampling profiler, python -m profiling.sampling (PEP 799).

Nothing in datasette/ is changed by this branch. The proposed changes are the patch files in patches/, one per optimisation, plus patches/ALL-combined.diff.

Headline results

ALL-combined.diff applies 48 patches to cf113a5. With it applied, the full test suite passes on 3.15: 2869 passed, which is the 2829 baseline tests plus 40 new ones. Black and ruff are clean.

These timings are in-process: the harness calls the ASGI app directly, with no HTTP server. The fixtures database is opened mutable (the datasette data.db default) with 3 SQL threads. Each figure is the median of 5 interleaved rounds. Base and patched trees were checked out at the same path depth.

Request	Before	After	Change
GET /favicon.ico	897 µs	31 µs	−97%
GET /-/static/app.css (83 KB)	4,141 µs	147 µs	−96%
GET /-/static/app.css?_hash=…, the URL every page links to	4,386 µs	200 µs	−95%
GET /-/versions (HTML)	3,420 µs	1,011 µs	−70%
GET /-/versions.json	1,935 µs	596 µs	−69%
GET /fixtures/nope/nope/nope/nope (404 page)	1,509 µs	484 µs	−68%
GET /- (301 redirect)	120 µs	39 µs	−67%
GET /-/actor.json with a ds_actor cookie	206 µs	80 µs	−61%
GET /-/static/app.css → 304 (If-None-Match)	274 µs	120 µs	−56%
GET /fixtures/ (trailing-slash redirect)	102 µs	50 µs	−51%
POST /-/actor.json (405)	118 µs	58 µs	−51%
GET /fixtures/facetable.json?_size=5…	6,794 µs	3,635 µs	−46%
GET /fixtures/facetable/1.json	3,031 µs	1,894 µs	−38%
GET /fixtures/compound_three_primary_keys?_size=100	48.8 ms	32.3 ms	−34%
GET /fixtures/facetable?_size=5 (HTML)	58.2 ms	43.0 ms	−26%
GET / (index HTML)	62.2 ms	49.8 ms	−20%
GET /…/compound_three_primary_keys.csv?_size=1000	21.6 ms	17.2 ms	−20%
GET /fixtures (database HTML)	65.5 ms	55.7 ms	−15%
GET /.json	53.7 ms	47.5 ms	−11%

The same fixtures opened as an immutable database (datasette -i):

Request	Before	After	Change
GET /fixtures/facetable.json?…	7,300 µs	1,981 µs	−73%
GET /fixtures/facetable/1.json	3,386 µs	1,094 µs	−68%
GET /fixtures	53.3 ms	23.8 ms	−55%
GET /fixtures/facetable	56.5 ms	28.0 ms	−50%

For the light endpoints, no patched round was slower than any base round. The heavy pages (tens of ms, dominated by SQL thread-pool round trips) vary ±10–15% between rounds, so their gains are less precise. The raw per-round numbers are in results/.

End-to-end results under a real datasette serve (uvicorn) are further down. There, the HTTP server layer is a large fixed cost, so the in-process percentages are diluted on light requests. Fixing the server stack is a lever of the same size.

Where the time went (baseline)

Share of wall-clock samples on the event-loop thread, from 10 kHz wall-mode profiles of the in-process harness at cf113a5 (results/baseline-profile-breakdown.txt):

Request	Biggest costs
Cookie-authenticated JSON (/-/actor.json)	re-parsing cookies, headers and query string 32%; itsdangerous signing 20%; GC 11%; dispatch (inspect, hooks, telemetry, routing) 10%
Anonymous redirect (/-)	dispatch 27% (17% is inspect.signature()); GC 16%; signing 10%; re-parsing 10%
404 page	Jinja 78%: walking every template directory, plus static() calling Path.resolve() 5–12 times
/-/versions.json	SQLite work on the event loop 53%: a new :memory: connection and FTS probes on every request
/-/static/app.css	waiting on the thread pool 81%: one aiofiles hop per 4 KB chunk
/fixtures/facetable.json	waiting on the thread pool 83%: 12 SQL round trips per request, most of them schema introspection

Most of the cost is re-doing work that never changes. Every access to request.cookies re-parses the Cookie header, and there are three per request. Every sign/unsign call builds a new serializer. Every request unsigns a ds_messages cookie that isn't there, which raises an exception and leaves a reference cycle for the GC. Every view call runs inspect.signature(). Every 404 walks every template directory. Every static asset is hashed or chunked in ways that cost far more than the bytes. None of this is new work per request; it is repeated work.

The proposed changes

Each patch applies on its own to cf113a5 with git apply, unless noted. They were found and prototyped by investigators working per area, with interleaved A/B timing, profiler evidence and the relevant tests. Each area was then reviewed adversarially for correctness, security and plugin compatibility. Where the reviewer asked for changes, the corrected version is the one in this directory.

"In-process effect" is from the final sequential run of each area's patches alone against the same base (results/inprocess-summary.txt). Effects below about ±8% are inside the noise of that run. Smaller wins are supported by micro-benchmarks and profiler sample shares in the area notes.

Request object and router — request-object-*, signing-*

Area effect: cookie-auth JSON −52%, redirect −39%, trailing slash −37%, POST 405 −44%, static 304 −35%, index HTML −14%.

Patch	What it does
request-object-1-cache-request-headers	Request.headers decoded every scope header on every access: 4–18 times per request. It is now cached per Request against a snapshot of scope["headers"], so it stays correct when route_path reassigns request.scope or a plugin replaces or appends headers. It still returns a fresh dict.
request-object-2-memoize-cookie-parse	Request.cookies built a new SimpleCookie on every access. Parsing is now memoised by Cookie-header string (LRU of 128, headers up to 8 KB only), and a copy is returned.
request-object-3-single-pass-send-wrapper	The Cache-Control/Vary privacy wrapper in route_path built request.headers twice and made four passes over the response headers. It now makes one pass. Output was fuzzed as byte-identical.
request-object-4-cache-request-args	Request.args re-ran parse_qs 29–34 times per table request. It is now cached per query string, with fresh lists on each access.
request-object-5-skip-absent-ds-messages-unsign	route_path unsigned the ds_messages cookie even when absent. unsign("") always raises BadSignature, and the traceback it left behind formed a reference cycle that triggered GC. (signing-2 is the same fix; apply only one.)
signing-1-cache-serializer	Datasette.sign/unsign built a new URLSafeSerializer per call: itsdangerous runs json.dumps({}) to sniff the serializer and re-derives the key. Serializers are now cached per (secret, namespace). Unhashable secrets, such as key-rotation lists, fall back to an uncached serializer.
signing-3-actor-cookie-single-parse	actor_from_request read request.cookies twice.

Routing and view dispatch — routing-*

Area effect: redirect −17%. datasette.client requests are 13–44% faster; see routing-2.

Patch	What it does
routing-1-cache-view-signatures	_gather_arguments called inspect.signature() on every request for every wrapped view and output renderer, and wrap_view_function called inspect.iscoroutinefunction per request. The parameter names are now cached in a WeakKeyDictionary, and the coroutine check moves to wrap time.
routing-2-reuse-default-routes	DatasetteClient.app calls ds.app() for every datasette.client request, which rebuilt about 61 routes and called get_plugins(). The default route table is now reused until the plugin registry or static_mounts changes. Plugin register_routes is still called every time. ds.app() drops from 318 to 5 µs.

Middleware, OpenTelemetry and GC — middleware-gc-*

Area effect: static 304 −11%, trailing slash −10%, index HTML −15%, index JSON −12%.

Patch	What it does
middleware-gc-1-telemetry-no-provider-fast-path	With no OpenTelemetry provider installed, TelemetryMiddleware still ran extract() and three nested no-op span context managers on every request, about 8 µs. It now skips the span when it would be INVALID_SPAN anyway.
middleware-gc-2-skip-noop-db-query-spans	Two no-op spans per SQL query (db.query and db.query.execute), about 10 µs of Python per query, which adds up on pages making 70+ queries. (Conflicts textually with patch 1; ALL-combined.diff resolves it.)
middleware-gc-3-csrftoken-no-scope-cycle	The legacy scope["csrftoken"] closure captured scope and was stored in it, so every request leaked a cycle to the GC. Combined with request-object-5, a request that makes no permission checks, such as /-/actor.json, now leaves zero cyclic garbage, down from 53 objects. Permission checks still leave about 13 each; see the bugs list.
middleware-gc-4-router-single-scope-copy	One dict(scope, …) copy per request instead of two, and a module-level import instead of one per request.
middleware-gc-5-telemetry-header-getter-length-check	The trace-context header getter lowercased every header name. It now compares lengths first.
middleware-gc-7-tracer-trace-fast-path	datasette.tracer.trace() looked up the asyncio task on every SQL query even with no ?_trace=1. Decorator use of @trace(...) keeps working.

Static files — static-files-*

Area effect: favicon −92%, app.css −94%, app.css?_hash= −95%, 404 page −47%.

Patch	What it does
static-files-1-cache-hash-param-sha256	A ?_hash= request, which is every static URL Datasette's templates emit, ran sha256_file() over the whole file synchronously on the event loop on every request. The hash is now cached per (inode, size, mtime_ns, ctime_ns).
static-files-2-send-file-fewer-thread-hops	asgi_send_file did a thread-pool hop for stat, open, close and every 4 KB read: about 25 hops and 21 ASGI messages for app.css. It now reads files up to 1 MiB in one hop and streams 64 KB chunks above that.
static-files-3-memoize-static-template-urls	The template static() helper resolved paths with Path.resolve() (about 50–90 µs) on every call, 5–12 times per HTML page. Successful results are now memoised.
static-files-4-favicon-from-memory	The favicon was re-read from disk, using 4 thread hops, on every request.
static-files-5-resolve-static-root-once	The static root was resolved again on every request. The first version of this patch was refuted in review, because a symlinked --static root re-pointed at runtime broke. This version re-resolves when the root's (st_dev, st_ino) changes and adds a regression test.
static-files-6-serve-small-static-files-from-memory	Files up to 1 MiB are served from a 16 MiB stat-validated in-memory cache, with zero thread hops. It keeps a running byte counter, after review found an O(n) re-sum on every cache miss.

Error paths — error-paths-*

Area effect: 404 page −20%. table_json −8% and permission checks generally are faster through patch 2.

Patch	What it does
error-paths-1-list-only-pages-templates	Every 404 called Environment.list_templates(), walking every template directory (the default one twice), to look for pages/{wildcard}.html. It now lists only pages/, with the same results and order on 19 loader shapes.
error-paths-2-permission-params-no-per-request-regex	PermissionSQL.allow() names each parameter :reason_N with a global counter, so check_permissions_for_actions compiled a never-seen regex for every permission check. It now does a single-pass rename.
error-paths-3-rich-traceback-only-for-5xx	Bug. With rich installed, every 4xx printed a show_locals=True traceback, about 0.9 s per table-not-found 404. Unmatched-route 404s crashed with ValueError (500), because print_exception ran outside an except block. Rich tracebacks now print only for 5xx errors, as the plain-Python path already does.

Template rendering plumbing — template-plumbing-*

Area effect: 100-row table page −16%, table HTML −7%. Render-only, the 100-row table page goes from 13.1 ms to 5.7 ms.

Patch	What it does
template-plumbing-1-buffer-table-rows-in-templates	In Jinja's async mode, every output fragment travels through async for event in gen: yield event at each extends/block/include level, about 170 ns per level per fragment. A 100-row table yields 6,810 fragments through 4–5 levels. The row loops in _table.html and _query_results.html are now captured with {% set %}…{% endset %}. Output is byte-identical on 47 endpoints. Users' overridden _table.html templates are unaffected.
template-plumbing-2-buffer-base-content-block	Does the same for base.html's content block, inside an empty {% with %} so the buffer variable cannot shadow a child template's or plugin's content_html. That collision was found in review.
template-plumbing-3-absolute-url-plain-str	absolute_url() passed a PrefixedUrlString to urljoin, costing about 58 µs per call.
template-plumbing-4-skip-render-cell-hook-without-impls	render_cell was dispatched once per cell even with no plugin implementing it. DATASETTE_TRACE_PLUGINS output is unchanged.
template-plumbing-5-hoist-table-path-out-of-row-loop	urls.table() was computed once per row.
template-plumbing-6-skip-template-hooks-without-impls	render_template called six hooks with no implementations on every page. This saves about 12 µs.
template-plumbing-7-database-view-reuse-selected-template	The database view selected its template twice.

Jinja async mode itself costs 35–43% on a render microbenchmark compared with sync mode. It cannot be turned off, because templates call async functions (crumb_items, menu_links) and the plugin docs promise that awaitables in template context are auto-awaited.

URL building — url-building-*

Area effect: table JSON −10%. Facet-heavy HTML pages improve 4–19% in the area's lower-noise harness.

Patch	What it does
url-building-1-prefixed-url-string-class-methods	PrefixedUrlString.__getattribute__ called dir(str) and built a closure on every attribute access, about 9.5 µs each, and urljoin makes about 7 per call. The wrapped methods are now defined once on the class. A property test over 180,000 calls gave identical results.
url-building-2-tilde-encode-safe-fast-path	tilde_encode ran a per-byte generator even for names that need no encoding. A regex fast path was checked identical on 200,000 strings.
url-building-3-tilde-decode-skip-token-when-nothing-to-decode	tilde_decode called secrets.token_hex(16), an os.urandom syscall, on every call, including for every database and table name in every URL.

Response serialization — response-serialization-*

These matter most under a real server, so the in-process harness shows little.

Patch	What it does
response-serialization-1-chunk-csv-stream-writes	CSV sent one ASGI message per row. A 20,000-row _stream=1 CSV was 20,003 messages. It now buffers up to 64K characters, flushed per page. Under uvicorn, a 1,000-row CSV goes from 45 ms to 17 ms and a 20k-row stream from 1.24 s to 0.67 s. Bytes are identical.
response-serialization-2-json-renderer-skip-infinity-scan	The JSON renderer scanned every value of every row in Python for infinities. It now encodes with allow_nan=False and falls back to the old path on ValueError. Behaviour change: _shape=arrayfirst with an infinity in a non-first column used to 500 (AssertionError) and now returns 200. A regression test is included.
response-serialization-3-csv-bytes-check-without-genexpr	Per-row any(isinstance(…) for …) becomes any(map(…)).
response-serialization-4-head-skips-csv-generation	HEAD on a CSV endpoint generated and streamed the whole CSV. For _stream=1 that meant paginating the whole table: 680 ms became 6 ms. Status and headers are identical.

DB-adjacent per-request work — db-adjacent-*

These are not data queries. They are introspection and probes that ran on every request. Area effect: /-/versions.json −50%, /-/versions −27%, table JSON −22%. On an immutable database, table JSON is −57%, row JSON −45% and the database page −45%.

Patch	What it does
db-adjacent-1-cache-versions-probe	/-/versions opened a :memory: connection, ran every prepare_connection hook and created three FTS virtual tables on every request. The result is now cached until the plugin set changes.
db-adjacent-2-memoize-detect-json1	detect_json1() with no connection opened a fresh SQLite connection on every table HTML page.
db-adjacent-3-column-types-skip-introspection	get_column_types ran PRAGMA table_xinfo even when no column types were assigned.
db-adjacent-4-reuse-table-columns-for-sort	table_columns() was introspected twice per table request.
db-adjacent-5-row-reuse-resolved-results	The row page ran the same primary-key SELECT twice.
db-adjacent-6-cache-immutable-db-introspection	For immutable databases the schema cannot change, so introspection is cached from one sqlite_master read. Review bounded the cache to real schema names, so junk ?_through= names cannot fill it.
db-adjacent-7-batch-refresh-schemas-reads	refresh_schemas() made 8 sequential round trips to read the internal catalog.
db-adjacent-8-skip-unused-label-column-lookups	table_view_data looked up a label column for every foreign key, then discarded the result.

Found by the completeness critic — critic-*

Patch	What it does
critic-1-permission-sql-stable-param-names	Because of the :reason_N counter, the SQL text of every permission check was unique, so SQLite re-prepared it every time: about 170 µs instead of 10 µs on the internal database. Parameters are now named by position. This needs error-paths-2 applied first. With error-paths-2 and critic-2, the effect was /-/versions.json −20%, row JSON −16% and table JSON −18%.
critic-2-table-rows-dict-zip	dict(sqlite3.Row) does a case-insensitive name lookup per column, which is O(columns²) per row. It now uses zip when names are unique.

End-to-end under datasette serve

These runs use a real datasette serve on the same 4-vCPU VM. A stdlib-only keep-alive load generator (tools/load.py) runs in separate processes with 8 connections. Each figure is the median of 5 interleaved rounds of 6 seconds. "Server CPU per request" is the server process's user+system CPU from /proc, divided by requests served, and is the steadier of the two metrics. Run it with tools/e2e_bench.py.

Endpoint	Server	req/s	Server CPU per request	vs base
/-/actor.json + cookie	cf113a5, stock serve (h11, access log)	1,493	659 µs	–
/-/actor.json + cookie	cf113a5 + httptools	2,162	461 µs	−30%
/-/actor.json + cookie	cf113a5, no access log	1,805	560 µs	−15%
/-/actor.json + cookie	All patches, stock serve	1,954	503 µs	−24%
/-/actor.json + cookie	All patches + httptools	3,053	324 µs	−51%
/-/actor.json + cookie	All patches + httptools, no access log, Content-Length	4,379	226 µs	−66%
/-/versions.json	cf113a5, stock serve (h11, access log)	412	2,944 µs	–
/-/versions.json	cf113a5 + httptools	446	2,761 µs	−6%
/-/versions.json	cf113a5, no access log	432	2,825 µs	−4%
/-/versions.json	All patches, stock serve	723	1,478 µs	−50%
/-/versions.json	All patches + httptools	847	1,304 µs	−56%
/-/versions.json	All patches + httptools, no access log, Content-Length	987	1,128 µs	−62%
/-/static/app.css	cf113a5, stock serve (h11, access log)	166	6,954 µs	–
/-/static/app.css	cf113a5 + httptools	190	6,034 µs	−13%
/-/static/app.css	cf113a5, no access log	172	6,708 µs	−4%
/-/static/app.css	All patches, stock serve	1,765	558 µs	−92%
/-/static/app.css	All patches + httptools	2,314	424 µs	−94%
/-/static/app.css	All patches + httptools, no access log, Content-Length	3,117	320 µs	−95%
/fixtures/facetable.json?_size=5	cf113a5, stock serve (h11, access log)	79	15,866 µs	–
/fixtures/facetable.json?_size=5	cf113a5 + httptools	80	15,952 µs	+1%
/fixtures/facetable.json?_size=5	cf113a5, no access log	77	16,133 µs	+2%
/fixtures/facetable.json?_size=5	All patches, stock serve	133	9,044 µs	−43%
/fixtures/facetable.json?_size=5	All patches + httptools	139	8,713 µs	−45%
/fixtures/facetable.json?_size=5	All patches + httptools, no access log, Content-Length	136	8,674 µs	−45%

Latency with a single connection (p50):

Endpoint	cf113a5	All patches	All patches + httptools, no access log
/-/actor.json + cookie	681 µs	582 µs	334 µs
/-/versions.json	2,403 µs	988 µs	772 µs
/-/static/app.css	5,924 µs	681 µs	489 µs
/fixtures/facetable.json?_size=5	6,393 µs	3,832 µs	3,781 µs

What this shows:

• The HTTP server is half the cost of a light request. Profiling the live server in attach mode showed uvicorn's layers taking 53–64% of server CPU on /-/actor.json with stock datasette serve: h11 parsing and serialising about 20%, socket I/O 15–18%, the access log 8–10%, and uvicorn's protocol code 8–11%. Datasette's ASGI app was 26–38% and GC 5–7%. So the patches' −61% in-process becomes −24% end to end on that request. After the patches the HTTP layer is about two-thirds of what is left.
• Datasette runs on pure-Python h11 by default. It depends on plain uvicorn, so httptools is never installed. With httptools installed, uvicorn's http="auto" picks it up with no code change. That alone cut server CPU per request by 30% on /-/actor.json, or 36% once the patches are applied. httptools 0.9.0 and uvloop 0.23.0 have cp315 wheels and install cleanly on 3.15.0b4.
• The uvicorn access log, which is always on under datasette serve, costs about 24 µs per request: 8–15% of a light request.
• Static files gain the most: 166 → 1,765 req/s with the patches on a stock server, and 3,117 req/s with the server changes too. The 4 KB thread-hop chunking was about 70% of static-asset cost under a real server, more than in-process.
• The table endpoint is limited by SQL thread round trips, so it gains from the DB-adjacent patches (79 → 133 req/s) and not from the HTTP server changes.
• The patched_best variant's Content-Length change is an experiment, monkeypatched in tools/serve_variant.py. It is not in ALL-combined.diff and contributes about 3%.

Bugs found along the way

Fixed by the patches above:

• rich makes 4xx responses very slow and makes unmatched routes crash (error-paths-3).
• HEAD on CSV URLs generates the entire CSV (response-serialization-4).
• ?_hash= static requests hash the whole file on the event loop (static-files-1).
• Every permission query is a fresh SQLite statement (critic-1).
• _shape=arrayfirst returns a 500 when a row holds an infinity outside the first column (response-serialization-2 changes this to 200).

Not fixed, reported for follow-up:

• httpx2 retries a failed import sniffio on every datasette.client request. sniffio is no longer installed, and a failed import is not cached, so sys.path finders are walked on every client and TestClient request. That is about 29% of a client request (454 → 324 µs when the failure is cached). Fix: depend on sniffio, or fix it upstream in httpx2.
• calculate_etag's cache is never invalidated. After a static file is edited in place, the old ETag keeps being served and clients get 304s for changed content. Keying it on (size, mtime_ns), as static-files-1 does for the hash, would fix it.
• tilde_decode is non-deterministic for input where a ~ escape is cut short by %. For example, tilde_decode("90~3%") returns 90% only about 63% of the time, because the random placeholder token gets decoded.
• --pdb with an unmatched-route 404 crashes in pdb.post_mortem(None).
• database_download sends both Content-Length and Transfer-Encoding: chunked.
• Foreign-key label links use tilde_encode(database_name) instead of the database route (views/table.py LINK_WITH_LABEL).
• Each permission check leaves about 13 cyclic objects through the self-recursive closures in Datasette._allowed_many.
• tests/test_plugins.py unregisters the wrong plugin around line 1547.

Further proposals, not in ALL-combined.diff

These are prototypes in patches/optional/. Each was measured, but either not adversarially reviewed or carrying a trade-off to decide first:

• Install httptools by default, for example through a datasette[fast] extra or uvicorn[standard]. See the end-to-end section. No code change.
• Add --no-access-log to datasette serve. The uvicorn access log is 8–13% of server CPU on light requests.
• Send Content-Length on non-streaming responses. About 3% end to end. It also gives clients sizes and progress.
• critic-3-extras-sequential: table HTML resolves about 30 "extras" as concurrent asyncio tasks (asyncinject parallel=True). Running them sequentially measured −18 to −20% on table HTML. This contradicts run_sequential in table.py (#2189), but the count and facet queries may genuinely benefit from overlapping on large tables.
• critic-5-request-scoped-introspection-memo: foreign keys are introspected 5×, and primary keys and PRAGMA schema_version 2×, per table HTML request. A request-scoped memo plus a one-trip prefetch measured −25% on table JSON. It does not yet clear the memo on writes within the same request.
• critic-6-inline-tableless-permission-checks: permission checks whose SQL reads no table could run inline (7 µs) instead of through the thread pool (330 µs). That would take /-/versions.json from 567 to 238 µs.
• critic-4-skip-fts-lookup-for-data-formats: one FTS-detection round trip per JSON/CSV table request that only HTML uses.
• middleware-gc-6-serve-gc-freeze-after-startup: gc.freeze() after startup removes about 40 ms gen-2 pauses on query-heavy pages. It made no difference end to end on light requests, and it hides startup objects from gc.get_objects(). Not recommended without an opt-out.

Ideas measured and rejected

• uvloop. datasette serve never uses it: cli.py calls asyncio.run(), which bypasses uvicorn's loop="auto". Forcing it gained nothing measurable.
• One combined alternation regex, or prefix dispatch, for routing. Route matching is 0.3–15 µs (2.5% of a row request). A combined regex is only 1.4× faster and unsafe for arbitrary plugin regexes.
• Sync Jinja. It is faster, but async template functions are a documented API.
• auto_reload=False, and caching negative template lookups. These would stop new or edited templates being picked up without a restart.
• GC threshold tuning. Within noise. GC came from cycles, which are now removed, not from heap size.
• Coalescing uvicorn's header and body writes. No gain, because only the first write of a response, the one that wakes the peer, is expensive.
• Reusing one httpx2.AsyncClient across datasette.client calls. It would leak cookies between calls.
• Running introspection on the event loop thread. A locked database would block the loop for up to the 5 s busy timeout.
• Overlapping thread-pool trips with asyncio.gather. This was 5–14% slower: more wake-ups and GIL contention.

The Python 3.15 sampling profiler

Every finding started from profiling.sampling. Examples:

# in-process steady-state loop, 10 kHz, collapsed stacks for analysis
python -m profiling.sampling run -r 10khz --mode wall --collapsed -o x.collapsed \
    tools/bench_asgi.py loop actor_json_cookie --seconds 6
python tools/analyze_collapsed.py x.collapsed --buckets
python tools/analyze_collapsed.py x.collapsed --children "DatasetteRouter.handle_404"

# attach to a running `datasette serve` while load runs
python -m profiling.sampling attach -r 10khz -d 10 --mode cpu --collapsed -o serve.collapsed PID

# which tree is a server running? dump shows file paths in the live stack
python -m profiling.sampling dump PID

These features were all useful: wall, cpu and gil modes; -a for all threads; --async-aware; --opcodes (with --heatmap/--flamegraph); --binary plus replay; --diff-flamegraph against a binary baseline; --heatmap; and --blocking. Sampling is out-of-process, so it slows the target very little, and attach works on a production-shaped server with no restart.

Things to know when reading its output on 3.15.0b4:

• Many samples fail in non-blocking mode: 25–65% "error rate", worst inside Jinja-compiled template code. Frames for included templates and child blocks often never appear, so template-body cost shows up on pass-through lines. --blocking drops errors to about 0.5% but slows the target about 3×. The layer split matched between the two within about 6 points.
• <GC> is attributed to whichever frame next hits the eval breaker, so it can appear under unrelated code such as Datasette.setting.
• run puts the current directory first on sys.path (like -m and -c). Run it from outside a source tree you don't intend to import.
• --opcodes is not accepted with --collapsed or --pstats.
• --async-aware --async-mode all failed with 100% errors on the table endpoint. It worked on lighter ones.
• Line numbers after --binary + replay were sometimes implausible, for example comment lines. Line-level shares here come from direct --collapsed runs.

Reproducing

uv python install 3.15
uv venv -p 3.15 /tmp/venv315
uv pip install -p /tmp/venv315 -e . pytest pytest-asyncio pytest-xdist beautifulsoup4 \
    pytest-timeout trustme cogapp opentelemetry-sdk "multipart-form-data-conformance==0.1a0"

# two trees at the same path depth: Datasette.static() calls Path.resolve(),
# which costs more for deeper install paths and skews HTML-page comparisons
git worktree add --detach ../t/base cf113a5
git worktree add --detach ../t/patched cf113a5
(cd ../t/patched && git apply "$OLDPWD"/perf/request-response/patches/ALL-combined.diff)

/tmp/venv315/bin/python perf/request-response/tools/measure_ab.py \
    --tree base=../t/base --tree patched=../t/patched \
    --endpoints actor_json_cookie,redirect_dash,versions_json,not_found,static_css \
    --n 1000 --rounds 5

tools/bench_asgi.py drives the ASGI app directly with realistic browser headers. tools/measure_ab.py interleaves trees round by round, using a fresh process per run. tools/analyze_collapsed.py attributes collapsed stacks to components. The measurements were taken on a 4-vCPU cloud VM, so compare ratios rather than absolute numbers.