to remember, bm_send_event() writes an event to the write cache, bm_flush_cache()
writes the contents of the write cache into the shared memory event buffer, buffer
free space is consumed. in the usual case, mlogger is reading events from the shared
memory event buffer, buffer free space is released. there is also a read cache, not
part of this discussion.
the purpose of the write cache is to reduce contention for the shared memory
semaphore. in the case of large number of small events, semaphore is locked per
cache-flush, instead of per-event. correct tuning of write cache and event size can
reduce lock rate from >100 kHz to around 100 Hz or lower.
for correct operation of bm_send_event() under all conditions we need to consider
all corner cases:
1) no write cache: (cache size set to 0)
- event_size > buffer_size -> reject the event (obviously)
- event_size > 0.5 * buffer_size -> only 1 event fits into the buffer, next write
will stall until mlogger reads the previous event (sequential operation, bad)
- event_size < 0.3 * buffer_size -> at least 2 events fit into the buffer (good)
decision: limit event size to 0.5 to 0.3 * buffer_size (current limit is 0.5 *
buffer_size, I think).
consequence: buffer size limit is 2 Gbytes (32-bit byte offsets, code is only 31-
bit-clean), max event size is between 1 Gbytes and 0.6 Gbytes.
2) writing to write cache:
- event_size > cache_size -> flush cache, write event to directly to buffer
- event_size > 0.5 * cache_size -> inefficient use of cache: write to cache, next
event does not fit, flush to buffer, repeat. no gain in semaphore locking (bad), one
additional memcpy() (event to cache and cache to buffer) (bad)
- event_size < 0.3 * cache_size -> multiple events fit into cache, but probably no
gain in semaphore locking
decision: events that are bigger than 0.3 to 0.1 * cache_size should not go through
the cache. (flush cache, write directly to buffer).
3) flush write cache to buffer:
- cache_size > buffer_size -> cannot flush in 1 operation, must have a loop and
flush the cache in pieces
- cache_size between 0.5 and 1.0 * buffer_size -> can flush in 1 operation, but must
wait for mlogger to fully empty the buffer (sequential operation, bad)
- cache size < 0.3 * buffer_size -> can flush in 1 operation, at least 2 "flushes"
fit inside the buffer (good)
decision: limit write cache size to 0.3 * buffer_size. (current limit is
- write cache size limit is 0.3..0.25 * 2GB = 0.6..0.5 Gbytes
- cached event size limit is 0.3..0.1 * 0.5 GBytes = 150..50 Mbytes
- minimum number of cached events: 3 to 10
- semaphore locks reduced: 3 to 10 locks become 1 lock (all events cached),
4 to 11 locks become 2 locks (big event causes cache flush).
- there is a periodic 1/second bm_flush_cache() that flushes the cache early and
reduces it's efficiency (but needed to avoid having data stuck in cache for long
- if multiple frontends use large write cache (~ 0.3..0.5 * buffer_size), again,
sequential operation can happen (bad)
- write cache is per-frontend, not per-equipment. if different equipments request
different cache sizes, mfe.c and tmfe c++ frontends complain about this, but the
user has to sort it out.