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reader_concurrency_semaphore: add metrics for shed reads

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When the admission queue capacity reaches its limits, excessive
reads are shed in order to avoid overload. Each such operation
now bumps the metrics, which can help the user judge if a replica
is overloaded.
This commit is contained in:
Piotr Sarna
2020-11-10 16:32:39 +01:00
parent 2e544a0c89
commit 3ce7848bdf
3 changed files with 18 additions and 0 deletions
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15
database.cc
View File

@@ -456,6 +456,11 @@ database::setup_metrics() {
" to be able to admit new ones, if there is a shortage of permits."),
{user_label_instance}),
sm::make_derive("reads_shed_due_to_overload", _read_concurrency_sem.get_stats().total_reads_shed_due_to_overload,
sm::description("The number of reads shed because the admission queue reached its max capacity."
" When the queue is full, excessive reads are shed to avoid overload."),
{user_label_instance}),
sm::make_gauge("active_reads", [this] { return max_count_streaming_concurrent_reads - _streaming_concurrency_sem.available_resources().count; },
sm::description("Holds the number of currently active read operations issued on behalf of streaming "),
{streaming_label_instance}),
@@ -481,6 +486,11 @@ database::setup_metrics() {
" to be able to admit new ones, if there is a shortage of permits."),
{streaming_label_instance}),
sm::make_derive("reads_shed_due_to_overload", _streaming_concurrency_sem.get_stats().total_reads_shed_due_to_overload,
sm::description("The number of reads shed because the admission queue reached its max capacity."
" When the queue is full, excessive reads are shed to avoid overload."),
{streaming_label_instance}),
sm::make_gauge("active_reads", [this] { return max_count_system_concurrent_reads - _system_read_concurrency_sem.available_resources().count; },
sm::description("Holds the number of currently active read operations from \"system\" keyspace tables. "),
{system_label_instance}),
@@ -505,6 +515,11 @@ database::setup_metrics() {
" to be able to admit new ones, if there is a shortage of permits."),
{system_label_instance}),
sm::make_derive("reads_shed_due_to_overload", _system_read_concurrency_sem.get_stats().total_reads_shed_due_to_overload,
sm::description("The number of reads shed because the admission queue reached its max capacity."
" When the queue is full, excessive reads are shed to avoid overload."),
{system_label_instance}),
sm::make_gauge("total_result_bytes", [this] { return get_result_memory_limiter().total_used_memory(); },
sm::description("Holds the current amount of memory used for results.")),

1
reader_concurrency_semaphore.cc
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@@ -436,6 +436,7 @@ bool reader_concurrency_semaphore::may_proceed(const resources& r) const {
future<reader_permit::resource_units> reader_concurrency_semaphore::do_wait_admission(reader_permit permit, size_t memory,
db::timeout_clock::time_point timeout) {
if (_wait_list.size() >= _max_queue_length) {
_stats.total_reads_shed_due_to_overload++;
if (_prethrow_action) {
_prethrow_action();
}

2
reader_concurrency_semaphore.hh
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@@ -91,6 +91,8 @@ public:
uint64_t total_successful_reads = 0;
// Total number of failed reads executed through this semaphore.
uint64_t total_failed_reads = 0;
// Total number of reads rejected because the admission queue reached its max capacity
uint64_t total_reads_shed_due_to_overload = 0;
};
struct permit_list;