Tuning Library Runtime Behavior

WORK IN PROGRESS

The following material is a work in progress and should not be considered complete or ready for public use.

Why?

No set of library defaults is appropriate for all workloads.

The GNU C Library makes assumptions on behalf of the user and provides a specific runtime behaviour that may not match the user workload or requirements.

For example the NPTL implementation sets a fixed cache size of 40MB for the re-use of thread stacks. Is it possible that this is correct under all workloads? Average workloads? This default was set 10 years ago and has not been revisited.

I propose we expose some of the library internals as tunable runtime parameters that our users and developers can use to tune the library. Developers would use them to achieve optimal mean performance for all users, while a single advanced user might use it to get the best performance from their application.

To reiterate:

We have immediate short-term needs today to expose library internals as tunable parameters, in particular:

How?

Examples

This is only a toy example of how one might use a global pointer, and a lockless algorithm, to push and pop tunable contexts for the entire library to use. The entire library would need to reference tunables via some levels of indirection through the global pointer (previously just referenced the global pointer).

For example: 

/* A definition of a tunable is a name/value tuple (for now).  */
struct __tunable {
  char *tunable;
  char *value;
};
typedef struct __tunable tunable;

/* The tunables have IDs that we use to index into the tunable table
   for each context.  */
enum {
  GNU_LIBC_PTHREAD_DEFAULT_STACKSIZE = 0,
  GNU_LIBC_PTHREAD_STACK_CACHESIZE = 1,
  ...
  GNU_LIBC_MAX_TUNABLE = 100
};

/* A context contains a set of tunables.  */
struct __tunable_context {
  char *id;
  tunable tlist[GNU_LIBC_MAX_TUNABLE];
  tunable_context *previous; 
};
typedef struct __tunable_context tunable_context;

/* Hidden pointer to active context in the library.  */
tunable_context *__default_tunable_context attribute_hidden;

/* Create a context from the current active context and call it ID.  */
tunable_context *create_tunable_context_np (const char *id);
int destroy_tunable_context_np (tunable_context *context);

/* Set a tunable for a context.  */
int set_tunable_np (tunable_context *context, const char *tunable, const char *value);
const char *get_tunable_np (tunable_context *context, const char *tunable);

/* Push or pop a context. Overrides the previous context.  */
int push_tunable_context_np (tunable_context *context);
tunable_context *pop_tunable_context_np (void);

/* Get the list of all tunables currently available.  */
int list_tunables_np (char **tunables, int *size);

e.g. 

tunable_context *ctx = create_tunable_context_np ();
if (set_tunable_np (ctx, "GNU_LIBC_PTHREAD_DEFAULT_STACKSIZE", "1048576") != 0)
  {
    /* Error handling.  */
  }
if (push_tunable_context_np (ctx) != 0)
  {
    /* Error handling.  */
  }
/* Do work with context active.  */
if (pop_tunable_context_np () == NULL)
  {
    /* Error handling.  */
  }
/* Restores previous context.  */

Per-process as an env var: 

export GNU_LIBC_$tunable=$value

Equivalent to calling the following at startup: 

tunable_context *ctx = create_tunable_context_np (NULL);
set_tunable_np (ctx, "GNU_LIBC_$tunable", "$value");
push_tunable_context_np (ctx);

Per-named-context as a env-var: 

export GNU_LIBC_$tunable_$id=$value

Equivalent to calling the following at startup: 

tunable_context *ctx = create_tunable_context_np ("$id");
set_tunable_np (ctx, "GNU_LIBC_$tunable", "$value");
push_tunable_context_np (ctx);

Where:

Notes: