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Docs: more details about concurrency, parallelism, threads

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Lovell Fuller 2022-04-25 14:40:55 +01:00
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23
docs/api-utility.md
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@ -95,7 +95,11 @@ Returns **[Object][1]**
## concurrency
Gets or, when a concurrency is provided, sets
the number of threads *libvips'* should create to process each image.
the maximum number of threads *libvips* should use to process *each image*.
These are from a thread pool managed by glib,
which helps avoid the overhead of creating new threads.
This method always returns the current concurrency.
The default value is the number of CPU cores,
except when using glibc-based Linux without jemalloc,
@ -103,10 +107,19 @@ where the default is `1` to help reduce memory fragmentation.
A value of `0` will reset this to the number of CPU cores.
The maximum number of images that can be processed in parallel
is limited by libuv's `UV_THREADPOOL_SIZE` environment variable.
Some image format libraries spawn additional threads,
e.g. libaom manages its own 4 threads when encoding AVIF images,
and these are independent of the value set here.
This method always returns the current concurrency.
The maximum number of images that sharp can process in parallel
is controlled by libuv's `UV_THREADPOOL_SIZE` environment variable,
which defaults to 4.
[https://nodejs.org/api/cli.html#uv_threadpool_sizesize][12]
For example, by default, a machine with 8 CPU cores will process
4 images in parallel and use up to 8 threads per image,
so there will be up to 32 concurrent threads.
### Parameters
@ -199,3 +212,5 @@ Returns **[boolean][10]**
[10]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Boolean
[11]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Number
[12]: https://nodejs.org/api/cli.html#uv_threadpool_sizesize

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docs/search-index.json
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21
lib/utility.js
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@ -103,7 +103,11 @@ cache(true);
/**
* Gets or, when a concurrency is provided, sets
* the number of threads _libvips'_ should create to process each image.
* the maximum number of threads _libvips_ should use to process _each image_.
* These are from a thread pool managed by glib,
* which helps avoid the overhead of creating new threads.
*
* This method always returns the current concurrency.
*
* The default value is the number of CPU cores,
* except when using glibc-based Linux without jemalloc,
@ -111,10 +115,19 @@ cache(true);
*
* A value of `0` will reset this to the number of CPU cores.
*
* The maximum number of images that can be processed in parallel
* is limited by libuv's `UV_THREADPOOL_SIZE` environment variable.
* Some image format libraries spawn additional threads,
* e.g. libaom manages its own 4 threads when encoding AVIF images,
* and these are independent of the value set here.
*
* This method always returns the current concurrency.
* The maximum number of images that sharp can process in parallel
* is controlled by libuv's `UV_THREADPOOL_SIZE` environment variable,
* which defaults to 4.
*
* https://nodejs.org/api/cli.html#uv_threadpool_sizesize
*
* For example, by default, a machine with 8 CPU cores will process
* 4 images in parallel and use up to 8 threads per image,
* so there will be up to 32 concurrent threads.
*
* @example
* const threads = sharp.concurrency(); // 4