Hello!

On Tue, Aug 07, 2018 at 02:26:01PM -0700, Ka-Hing Cheung via nginx-devel wrote:

> commit 0bcfefa040abdff674047c15701d237ff16a5f2b
> Author: Ka-Hing Cheung <kahing@cloudflare.com>
> Date: Fri Aug 3 13:37:58 2018 -0700
>
> move open to thread pool
>
> At cloudflare we found that open() can block a long time, especially
> at p99 and p999. Moving it to thread pool improved overall p99 by 6x
> or so during peak time.
>
> This has the side effect of disabling nginx's open_file_cache when
> "aio threads" is turned on. For us we found that to have no impact
> (probably because we have way too many files for open file cache to
> be effective anyway).
>
> thread pool does increase CPU usage somewhat but we have not measured
> difference in CPU usage for stock "aio threads" compared to after this
> patch.
>
> Only the cache hit open() is moved to thread pool.
>
> We wrote more about it here:
> https://blog.cloudflare.com/how-we-scaled-nginx-and-saved-the-world-54-years-every-day/

First of all, thank you for your work. The numbers provided in
the article looks promising.

We've thought about offloading more operations to thread pools,
and that's basically why "aio_write" is a separate directive, so
more directives like "aio_<some_operation>" can be added in the
future. It's good to see this is finally happening.

Unfortunately, there are number problems with the patch, and it
needs more work before it can be committed. In particular:

- The open() calls are offloaded to thread pools unconditionally.
This may actually reduce performance in various typical
workloads where open() does not block. Instead, there should be
an "aio_open on|off" directive, similar to "aio_write".

- The code bypasses open file cache, and uses a direct call
in the http cache code instead. While it might be ok in your
setup, it looks like an incomplete solution from the generic point
of view. A better solution would be to introduce a generic
interface in ngx_open_cached_file() to allow use of thread
pools.

- The code calls ngx_open_and_stat_file() whithin a thread, which
is relatively complex function never designed to be thread safe.
While it looks like currently it is, a better solution would be to
introduce a separate simple function to execute within a thread,
similar to ngx_thread_read_handler().

- In the ngx_http_file_cache_thread_open() function you allocate
thread task of size sizeof(ngx_thread_file_open_ctx_t) and store
it in the file->thread_task. There is no guarantee that this
task will be compatible with other tasks stored in
file->thread_task. A better solution would be to extend
ngx_thread_file_ctx_t and use the same context for all
file-related threaded operations.

- A better place for the thread-specific code would be
src/os/unix/ngx_files.c, where ngx_thread_file_ctx_t and
existing threaded file operations are defined.

- The code uses memory pool operations wihin a thread,
specifically ngx_pool_cleanup_add(). Memory pools are not
thread safe, and the behaviour of this code is therefore
undefined.

- Also there are various style issues, including wrong
capitalization of the commit log summary line, strange commit
log indentation and header, missing empty lines. Please take a
look at http://nginx.org/en/docs/contributing_changes.html for
various hints. In particular, you may want to use Mercurial to
submit patches.

See also below for some in-line comments.

>
> diff --git src/core/ngx_open_file_cache.c src/core/ngx_open_file_cache.c
> index b23ee78d..9177ebfd 100644
> --- src/core/ngx_open_file_cache.c
> +++ src/core/ngx_open_file_cache.c
> @@ -35,8 +35,6 @@ static ngx_fd_t ngx_open_file_wrapper(ngx_str_t *name,
> ngx_int_t access, ngx_log_t *log);
> static ngx_int_t ngx_file_info_wrapper(ngx_str_t *name,
> ngx_open_file_info_t *of, ngx_file_info_t *fi, ngx_log_t *log);
> -static ngx_int_t ngx_open_and_stat_file(ngx_str_t *name,
> - ngx_open_file_info_t *of, ngx_log_t *log);
> static void ngx_open_file_add_event(ngx_open_file_cache_t *cache,
> ngx_cached_open_file_t *file, ngx_open_file_info_t *of, ngx_log_t *log);
> static void ngx_open_file_cleanup(void *data);
> @@ -836,7 +834,7 @@ ngx_file_info_wrapper(ngx_str_t *name,
> ngx_open_file_info_t *of,
> }
>
>
> -static ngx_int_t
> +ngx_int_t
> ngx_open_and_stat_file(ngx_str_t *name, ngx_open_file_info_t *of,
> ngx_log_t *log)
> {
> diff --git src/core/ngx_open_file_cache.h src/core/ngx_open_file_cache.h
> index d119c129..94b4d552 100644
> --- src/core/ngx_open_file_cache.h
> +++ src/core/ngx_open_file_cache.h
> @@ -124,6 +124,8 @@ ngx_open_file_cache_t
> *ngx_open_file_cache_init(ngx_pool_t *pool,
> ngx_uint_t max, time_t inactive);
> ngx_int_t ngx_open_cached_file(ngx_open_file_cache_t *cache, ngx_str_t *name,
> ngx_open_file_info_t *of, ngx_pool_t *pool);
> +ngx_int_t ngx_open_and_stat_file(ngx_str_t *name,
> + ngx_open_file_info_t *of, ngx_log_t *log);
>
>
> #endif /* _NGX_OPEN_FILE_CACHE_H_INCLUDED_ */
> diff --git src/http/ngx_http_cache.h src/http/ngx_http_cache.h
> index f9e96640..2910f6a1 100644
> --- src/http/ngx_http_cache.h
> +++ src/http/ngx_http_cache.h
> @@ -114,6 +114,7 @@ struct ngx_http_cache_s {
> unsigned exists:1;
> unsigned temp_file:1;
> unsigned purged:1;
> + unsigned opening:1;
> unsigned reading:1;
> unsigned secondary:1;
> unsigned background:1;
> diff --git src/http/ngx_http_file_cache.c src/http/ngx_http_file_cache.c
> index 56866fa4..9cb110bf 100644
> --- src/http/ngx_http_file_cache.c
> +++ src/http/ngx_http_file_cache.c
> @@ -18,6 +18,10 @@ static void
> ngx_http_file_cache_lock_wait(ngx_http_request_t *r,
> ngx_http_cache_t *c);
> static ngx_int_t ngx_http_file_cache_read(ngx_http_request_t *r,
> ngx_http_cache_t *c);
> +static ngx_int_t ngx_http_file_cache_aio_open(ngx_http_request_t *r,
> + ngx_http_cache_t *c, ngx_int_t *rv);
> +static ngx_int_t ngx_http_file_cache_do_aio_open(ngx_http_request_t *r,
> + ngx_http_cache_t *c, ngx_open_file_info_t *of, ngx_int_t *rv);
> static ssize_t ngx_http_file_cache_aio_read(ngx_http_request_t *r,
> ngx_http_cache_t *c);
> #if (NGX_HAVE_FILE_AIO)
> @@ -268,9 +272,7 @@ ngx_http_file_cache_open(ngx_http_request_t *r)
> ngx_uint_t test;
> ngx_http_cache_t *c;
> ngx_pool_cleanup_t *cln;
> - ngx_open_file_info_t of;
> ngx_http_file_cache_t *cache;
> - ngx_http_core_loc_conf_t *clcf;
>
> c = r->cache;
>
> @@ -278,6 +280,10 @@ ngx_http_file_cache_open(ngx_http_request_t *r)
> return NGX_AGAIN;
> }
>
> + if (c->opening) {
> + return ngx_http_file_cache_aio_open(r, c, &rv);
> + }
> +

The value of rv is uninitialized here. While it is not strictly a
bug, but this code indicate bad design. Also, duplicate handling
of "rv == NGX_DECLINED" below and in the
ngx_http_file_cache_aio_open() function contributes to the
problem, as well as passing the rv value to the threaded operation
where it is not needed. It might be a good idea to re-think the
design.

> if (c->reading) {
> return ngx_http_file_cache_read(r, c);
> }
> @@ -343,23 +349,33 @@ ngx_http_file_cache_open(ngx_http_request_t *r)
> goto done;
> }
>
> - clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
> + return ngx_http_file_cache_aio_open(r, c, &rv);
>
> - ngx_memzero(&of, sizeof(ngx_open_file_info_t));
> +done:
>
> - of.uniq = c->uniq;
> - of.valid = clcf->open_file_cache_valid;
> - of.min_uses = clcf->open_file_cache_min_uses;
> - of.events = clcf->open_file_cache_events;
> - of.directio = NGX_OPEN_FILE_DIRECTIO_OFF;
> - of.read_ahead = clcf->read_ahead;
> + if (rv == NGX_DECLINED) {
> + return ngx_http_file_cache_lock(r, c);
> + }
>
> - if (ngx_open_cached_file(clcf->open_file_cache, &c->file.name,
> &of, r->pool)
> - != NGX_OK)
> - {
> - switch (of.err) {
> + return rv;
> +}
> +
> +static ngx_int_t

Style: there should be two empty lines between functions.

> +ngx_http_file_cache_aio_open(ngx_http_request_t *r, ngx_http_cache_t *c,
> + ngx_int_t *rv)

Style: function arguments should be indented with 4 spaces.

> +{
> + ngx_int_t rc;
> + ngx_open_file_info_t of;
> + ngx_http_file_cache_t *cache = c->file_cache;
>
> - case 0:
> + rc = ngx_http_file_cache_do_aio_open(r, c, &of, rv);
> + if (rc == NGX_AGAIN) {
> + return rc;
> + }
> +
> + if (rc != NGX_OK) {
> + switch (of.err) {
> + case NGX_OK:
> return NGX_ERROR;
>
> case NGX_ENOENT:
> @@ -391,14 +407,13 @@ ngx_http_file_cache_open(ngx_http_request_t *r)
>
> done:
>
> - if (rv == NGX_DECLINED) {
> + if (*rv == NGX_DECLINED) {
> return ngx_http_file_cache_lock(r, c);
> }
>
> - return rv;
> + return *rv;
> }
>
> -
> static ngx_int_t

Style: there should be two empty lines between functions:

> ngx_http_file_cache_lock(ngx_http_request_t *r, ngx_http_cache_t *c)
> {
> @@ -663,6 +678,127 @@ ngx_http_file_cache_read(ngx_http_request_t *r,
> ngx_http_cache_t *c)
> }
>
>
> +#if (NGX_THREADS)
> +typedef struct {

Style: as long as #if contains something complex, it should be
separated with empty lines.

> + ngx_str_t name;
> + ngx_pool_t *pool;
> + ngx_open_file_info_t of;
> + ngx_int_t rv;
> +} ngx_thread_file_open_ctx_t;
> +
> +static void
> +ngx_http_file_cache_thread_open_handler(void *data, ngx_log_t *log)
> +{
> + ngx_thread_file_open_ctx_t *ctx = data;
> + ngx_pool_t *pool = ctx->pool;
> + ngx_open_file_info_t *of = &ctx->of;
> + ngx_pool_cleanup_t *cln;
> + ngx_pool_cleanup_file_t *clnf;
> + ngx_int_t rc;

Style: variable types should be sorted from shortest to longest;
variable names should be separated with two spaces from the
longest type; assignments should be written separately, except
some specific cases where one assignment is allowed in a separate
variables block.

> +
> + ngx_log_debug0(NGX_LOG_DEBUG_CORE, log, 0, "thread read handler");

The "read" here is incorrect, should be "open".

> +
> + cln = ngx_pool_cleanup_add(pool, sizeof(ngx_pool_cleanup_file_t));
> + if (cln == NULL) {
> + ctx->of.err = NGX_ERROR;
> + return;
> + }

The code is executed in a thread, and using request pool here is
not allowed.

> +
> + of->fd = NGX_INVALID_FILE;
> +
> + rc = ngx_open_and_stat_file(&ctx->name, of, pool->log);
> + if (rc == NGX_OK && !of->is_dir) {
> + cln->handler = ngx_pool_cleanup_file;
> + clnf = cln->data;
> +
> + clnf->fd = of->fd;
> + clnf->name = ctx->name.data;
> + clnf->log = pool->log;
> + }
> +}
> +
> +
> +static ngx_int_t
> +ngx_http_file_cache_thread_open(ngx_http_cache_t *c, ngx_open_file_info_t *of,
> + ngx_int_t *rv, ngx_pool_t *pool)
> +{
> + ngx_thread_task_t *task;
> + ngx_thread_file_open_ctx_t *ctx;
> + ngx_file_t *file = &c->file;

Style, see above.

> +
> + task = file->thread_task;
> +
> + if (task == NULL) {
> + task = ngx_thread_task_alloc(pool, sizeof(ngx_thread_file_open_ctx_t));
> + if (task == NULL) {
> + return NGX_ERROR;
> + }
> +
> + file->thread_task = task;
> + }

See above, this needs some guarantee that what's stored in
file->thread_task is compatible with all operations.

> +
> + ctx = task->ctx;
> +
> + if (task->event.complete) {
> + task->event.complete = 0;
> +
> + *of = ctx->of;
> + *rv = ctx->rv;
> + return of->err;
> + }
> +
> + task->handler = ngx_http_file_cache_thread_open_handler;
> +
> + ctx->pool = pool;
> + ctx->name = file->name;
> + ctx->of = *of;
> + ctx->rv = *rv;
> +
> + if (file->thread_handler(task, file) != NGX_OK) {
> + return NGX_ERROR;
> + }
> +
> + return NGX_AGAIN;
> +}
> +#endif
> +
> +static ngx_int_t

Style: there should be an empty line before "#endif", and two
empty lines between "#endif" and the next function.

> +ngx_http_file_cache_do_aio_open(ngx_http_request_t *r, ngx_http_cache_t *c,
> + ngx_open_file_info_t *of, ngx_int_t *rv)
> +{
> + ngx_http_core_loc_conf_t *clcf;
> +
> + clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
> +
> + ngx_memzero(of, sizeof(ngx_open_file_info_t));
> +
> + of->uniq = c->uniq;
> + of->valid = clcf->open_file_cache_valid;
> + of->min_uses = clcf->open_file_cache_min_uses;
> + of->events = clcf->open_file_cache_events;
> + of->directio = NGX_OPEN_FILE_DIRECTIO_OFF;
> + of->read_ahead = clcf->read_ahead;
> +
> +#if (NGX_THREADS)
> + if (clcf->aio == NGX_HTTP_AIO_THREADS) {
> + ngx_int_t rc;

Style: there should be two spaces between a type and a variable
name; there should be an empty line after #if (and before #endif).

> +
> + c->file.thread_task = c->thread_task;
> + c->file.thread_handler = ngx_http_cache_thread_handler;
> + c->file.thread_ctx = r;
> +
> + rc = ngx_http_file_cache_thread_open(c, of, rv, r->pool);
> +
> + c->thread_task = c->file.thread_task;
> + c->opening = (rc == NGX_AGAIN);
> +
> + return rc;
> + }
> +#endif
> +
> + return ngx_open_cached_file(clcf->open_file_cache, &c->file.name,
> of, r->pool);

Style: maximum text width is 80 characters.

> +}
> +
> static ssize_t
> ngx_http_file_cache_aio_read(ngx_http_request_t *r, ngx_http_cache_t *c)
> {
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> http://mailman.nginx.org/mailman/listinfo/nginx-devel

--
Maxim Dounin
http://mdounin.ru/
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