Friday, December 19, 2014

Memory Management in H2O

This blogpost (as part of the H2O Advent Calendar 2014) provides a high-level overview of the memory management functions in H2O that can be categorized into four groups.

h2o_mem_alloc, h2o_mem_realloc

They are wrappers of malloc(3) / realloc(3), that calls abort(3) if memory allocation fails. The returned chunks should be freed by calling free(3).

h2o_mem_init_pool, h2o_mem_clear_pool, h2o_mem_alloc_pool

The functions create, clear, and allocate from a memory pool. The term memory pool has several meanings, but in case of H2O the term has been borrowed from Apache; it refers to a memory allocator that frees all associated chunks at once when the destructor (h2o_mem_clear_pool) is being called.

The primary use-case of the functions is to allocate memory that relates to a HTTP request. The request object h2o_req_t has a memory pool associated to it; small chunks of memory that need to be allocated while handling a request should be obtained by calling h2o_mem_alloc_pool instead of h2o_mem_alloc, since the former is generally faster than the latter.

h2o_mem_alloc_shared, h2o_mem_link_shared, h2o_mem_addref_shared, h2o_mem_release_shared

They are the functions to handle ref-counted chunks of memory. Eeach shared chunk has its own dispose callback that gets called when the reference counter reaches zero. A chunk can be optionally associated to a memory pool, so that the reference counter gets decremented when the pool gets flushed.

The functions are used for handling things like headers transferred via HTTP/2, or to for associating a resource that needs a custom dispose callback to a HTTP request through the use of the memory pool.

h2o_buffer_init, h2o_buffer_dispose, h2o_buffer_reserve, h2o_buffer_consume, h2o_buffer_link_to_pool

The functions provide access to buffer, that can hold any length of octets. They internally use malloc(3) / realloc(3) for handling short buffers, and switch to using temporary-file-backed mmap(2) when the length of the buffer reaches a predefined threshold (default: 32MB). A buffer can also be associated to memory pool by calling the h2o_buffer_link_to_pool function.

The primary use-case of the buffer is to store incoming HTTP requests and POST contents (as it can be used to hold huge chunks on 64-bit systems since it switches to temporary-file-backed memory as described).


The function reserves given number of slots for H2O_VECTOR which is a variable length array of an arbitrary type of data. Either h2o_mem_realloc or the memory pool can be used as the underlying memory allocator (in the former case, the allocated memory should be manually freed by the caller). The structure is initialized by zero-filling it.

The vector is used everywhere, from storing a list of HTTP headers to a list of configuration directives.

For details, please refer to their doc-comment and the definitions in include/h2o/memory.h and lib/memory.c.


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