doc: threadpool size, and APIs using the pool

Not knowing which APIs use libuv's threadpool can lead to surprising
performance problems. Document the APIs, and also document
UV_THREADPOOL_SIZE, which can be used to fix problems.

PR-URL: https://github.com/nodejs/node/pull/14995
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/cli.md

@@ -568,6 +568,30 @@ appended to if it does. If an error occurs while attempting to write the
 warning to the file, the warning will be written to stderr instead. This is
 equivalent to using the `--redirect-warnings=file` command-line flag.
 
+### `UV_THREADPOOL_SIZE=size`
+
+Set the number of threads used in libuv's threadpool to `size` threads.
+
+Asynchronous system APIs are used by Node.js whenever possible, but where they
+do not exist, libuv's threadpool is used to create asynchronous node APIs based
+on synchronous system APIs. Node.js APIs that use the threadpool are:
+
+- all `fs` APIs, other than the file watcher APIs and those that are explicitly
+  synchronous
+- `crypto.pbkdf2()`
+- `crypto.randomBytes()`, unless it is used without a callback
+- `crypto.randomFill()`
+- `dns.lookup()`
+- all `zlib` APIs, other than those that are explicitly synchronous
+
+Because libuv's threadpool has a fixed size, it means that if for whatever
+reason any of these APIs takes a long time, other (seemingly unrelated) APIs
+that run in libuv's threadpool will experience degraded performance. In order to
+mitigate this issue, one potential solution is to increase the size of libuv's
+threadpool by setting the `'UV_THREADPOOL_SIZE'` environment variable to a value
+greater than `4` (its current default value).  For more information, see the
+[libuv threadpool documentation][].
+
 [`--openssl-config`]: #cli_openssl_config_file
 [Buffer]: buffer.html#buffer_buffer
 [Chrome Debugging Protocol]: https://chromedevtools.github.io/debugger-protocol-viewer
@@ -575,3 +599,4 @@ equivalent to using the `--redirect-warnings=file` command-line flag.
 [SlowBuffer]: buffer.html#buffer_class_slowbuffer
 [debugger]: debugger.html
 [emit_warning]: process.html#process_process_emitwarning_warning_type_code_ctor
+[libuv threadpool documentation]: http://docs.libuv.org/en/latest/threadpool.html

doc/api/crypto.md

@@ -1559,6 +1559,10 @@ crypto.pbkdf2('secret', 'salt', 100000, 512, 'sha512', (err, derivedKey) => {
 An array of supported digest functions can be retrieved using
 [`crypto.getHashes()`][].
 
+Note that this API uses libuv's threadpool, which can have surprising and
+negative performance implications for some applications, see the
+[`UV_THREADPOOL_SIZE`][] documentation for more information.
+
 ### crypto.pbkdf2Sync(password, salt, iterations, keylen, digest)
 <!-- YAML
 added: v0.9.3
@@ -1723,6 +1727,10 @@ This should normally never take longer than a few milliseconds. The only time
 when generating the random bytes may conceivably block for a longer period of
 time is right after boot, when the whole system is still low on entropy.
 
+Note that this API uses libuv's threadpool, which can have surprising and
+negative performance implications for some applications, see the
+[`UV_THREADPOOL_SIZE`][] documentation for more information.
+
 ### crypto.randomFillSync(buffer[, offset][, size])
 <!-- YAML
 added: v7.10.0
@@ -1783,6 +1791,10 @@ crypto.randomFill(buf, 5, 5, (err, buf) => {
 });
 ```
 
+Note that this API uses libuv's threadpool, which can have surprising and
+negative performance implications for some applications, see the
+[`UV_THREADPOOL_SIZE`][] documentation for more information.
+
 ### crypto.setEngine(engine[, flags])
 <!-- YAML
 added: v0.11.11
@@ -2208,6 +2220,7 @@ the `crypto`, `tls`, and `https` modules and are generally specific to OpenSSL.
 
 [`Buffer`]: buffer.html
 [`EVP_BytesToKey`]: https://www.openssl.org/docs/man1.0.2/crypto/EVP_BytesToKey.html
+[`UV_THREADPOOL_SIZE`]: cli.html#cli_uv_threadpool_size_size
 [`cipher.final()`]: #crypto_cipher_final_outputencoding
 [`cipher.update()`]: #crypto_cipher_update_data_inputencoding_outputencoding
 [`crypto.createCipher()`]: #crypto_crypto_createcipher_algorithm_password

doc/api/dns.md

@@ -620,15 +620,16 @@ but note that changing these files will change the behavior of _all other
 programs running on the same operating system_.
 
 Though the call to `dns.lookup()` will be asynchronous from JavaScript's
-perspective, it is implemented as a synchronous call to getaddrinfo(3) that
-runs on libuv's threadpool. Because libuv's threadpool has a fixed size, it
-means that if for whatever reason the call to getaddrinfo(3) takes a long
-time, other operations that could run on libuv's threadpool (such as filesystem
-operations) will experience degraded performance. In order to mitigate this
-issue, one potential solution is to increase the size of libuv's threadpool by
-setting the `'UV_THREADPOOL_SIZE'` environment variable to a value greater than
-`4` (its current default value). For more information on libuv's threadpool, see
-[the official libuv documentation][].
+perspective, it is implemented as a synchronous call to getaddrinfo(3) that runs
+on libuv's threadpool. This can have surprising negative performance
+implications for some applications, see the [`UV_THREADPOOL_SIZE`][]
+documentation for more information.
+
+Note that various networking APIs will call `dns.lookup()` internally to resolve
+host names. If that is an issue, consider resolving the hostname to and address
+using `dns.resolve()` and using the address instead of a host name. Also, some
+networking APIs (such as [`socket.connect()`][] and [`dgram.createSocket()`][])
+allow the default resolver, `dns.lookup()`, to be replaced.
 
 ### `dns.resolve()`, `dns.resolve*()` and `dns.reverse()`
 
@@ -644,6 +645,8 @@ They do not use the same set of configuration files than what [`dns.lookup()`][]
 uses. For instance, _they do not use the configuration from `/etc/hosts`_.
 
 [`Error`]: errors.html#errors_class_error
+[`UV_THREADPOOL_SIZE`]: cli.html#cli_uv_threadpool_size_size
+[`dgram.createSocket()`]: dgram.html#dgram_dgram_createsocket_options_callback
 [`dns.getServers()`]: #dns_dns_getservers
 [`dns.lookup()`]: #dns_dns_lookup_hostname_options_callback
 [`dns.resolve()`]: #dns_dns_resolve_hostname_rrtype_callback
@@ -660,6 +663,7 @@ uses. For instance, _they do not use the configuration from `/etc/hosts`_.
 [`dns.resolveTxt()`]: #dns_dns_resolvetxt_hostname_callback
 [`dns.reverse()`]: #dns_dns_reverse_ip_callback
 [`dns.setServers()`]: #dns_dns_setservers_servers
+[`socket.connect()`]: net.html#net_socket_connect_options_connectlistener
 [`util.promisify()`]: util.html#util_util_promisify_original
 [DNS error codes]: #dns_error_codes
 [Implementation considerations section]: #dns_implementation_considerations

doc/api/fs.md

@@ -100,6 +100,13 @@ example `fs.readdirSync('c:\\')` can potentially return a different result than
 `fs.readdirSync('c:')`. For more information, see
 [this MSDN page][MSDN-Rel-Path].
 
+## Threadpool Usage
+
+Note that all file system APIs except `fs.FSWatcher()` and those that are
+explicitly synchronous use libuv's threadpool, which can have surprising and
+negative performance implications for some applications, see the
+[`UV_THREADPOOL_SIZE`][] documentation for more information.
+
 ## WHATWG URL object support
 <!-- YAML
 added: v7.6.0
@@ -2845,6 +2852,7 @@ The following constants are meant for use with the [`fs.Stats`][] object's
 [`ReadDirectoryChangesW`]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365465%28v=vs.85%29.aspx
 [`ReadStream`]: #fs_class_fs_readstream
 [`URL`]: url.html#url_the_whatwg_url_api
+[`UV_THREADPOOL_SIZE`]: cli.html#cli_uv_threadpool_size_size
 [`WriteStream`]: #fs_class_fs_writestream
 [`event ports`]: http://illumos.org/man/port_create
 [`fs.FSWatcher`]: #fs_class_fs_fswatcher

doc/api/zlib.md

@@ -43,6 +43,13 @@ zlib.unzip(buffer, (err, buffer) => {
 });
 ```
 
+## Threadpool Usage
+
+Note that all zlib APIs except those that are explicitly synchronous use libuv's
+threadpool, which can have surprising and negative performance implications for
+some applications, see the [`UV_THREADPOOL_SIZE`][] documentation for more
+information.
+
 ## Compressing HTTP requests and responses
 
 The `zlib` module can be used to implement support for the `gzip` and `deflate`