[tarantool-patches] [PATCH v3 09/14] wal: a dedicated wal scheduling fiber
Vladimir Davydov
vdavydov.dev at gmail.com
Thu Jun 13 17:24:52 MSK 2019
On Sun, Jun 09, 2019 at 11:44:38PM +0300, Georgy Kirichenko wrote:
> In order to implement asynchronous transaction we need to run a
> transaction completion handler but tx_prio is not able to yield.
>
> Prerequisites: #1254
> ---
> src/box/wal.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 41 insertions(+), 7 deletions(-)
>
> diff --git a/src/box/wal.c b/src/box/wal.c
> index 86d021896..e868a8e71 100644
> --- a/src/box/wal.c
> +++ b/src/box/wal.c
> @@ -100,6 +100,12 @@ struct wal_writer
> struct cpipe wal_pipe;
> /** A memory pool for messages. */
> struct mempool msg_pool;
> + /** A queue to schedule journal entry completions. */
> + struct stailq schedule_queue;
> + /** True if writer is in rollback state. */
> + bool is_in_rollback;
> + /** A condition to signal about new schedule queue entries. */
> + struct fiber_cond schedule_cond;
> /* ----------------- wal ------------------- */
> /** A setting from instance configuration - rows_per_wal */
> int64_t wal_max_rows;
> @@ -254,17 +260,36 @@ xlog_write_entry(struct xlog *l, struct journal_entry *entry)
> return xlog_tx_commit(l);
> }
>
> +/*
> + * Tx schedule fiber function.
> + */
> +static int
> +tx_schedule_f(va_list ap)
> +{
> + struct wal_writer *writer = va_arg(ap, struct wal_writer *);
> + while (!fiber_is_cancelled()) {
> + while (!stailq_empty(&writer->schedule_queue)) {
> + struct journal_entry *req =
> + stailq_shift_entry(&writer->schedule_queue,
> + struct journal_entry, fifo);
> + req->done = true;
> + fiber_cond_broadcast(&req->done_cond);
> + }
> + writer->is_in_rollback = false;
> + fiber_cond_wait(&writer->schedule_cond);
> + }
> + return 0;
> +}
> +
> /**
> - * Signal done condition.
> + * Attach requests to a scheduling queue.
> */
> static void
> tx_schedule_queue(struct stailq *queue)
> {
> - struct journal_entry *req;
> - stailq_foreach_entry(req, queue, fifo) {
> - req->done = true;
> - fiber_cond_broadcast(&req->done_cond);
> - }
> + struct wal_writer *writer = &wal_writer_singleton;
> + stailq_concat(&writer->schedule_queue, queue);
> + fiber_cond_signal(&writer->schedule_cond);
This adds an extra ctxsw to the relatively hot WAL writer wakeup path so
we can't commit it. I assume it's a temporary hack you added so that you
can submit this patches for review.
I understand why you're doing this, but the right way would be removing
yields from on_commit triggers. We need to make vy_log_write yield-free.
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