Various performance and stability improvements, hash/data table separation
major changes ============= hash/data table --------------- The hash table is now only used for _OPEN records, and the actual data is stored in a data table. Upon submit, hash entries are cleared and data continues to live in the data table until it gets freed by a worker (or upon submit if it is a NODATA record). This drastically reduces the hash table load and significantly increases worst case performance. In particular, the hash table load is now independend of ActiveMQ backend performance (read: stalls). Preliminary recommendations fon table sizing: * hash table: double max_sessions from varnish e.g. maxopen.scale = 16 for 64K hash table entries to support >32K sessions (savely and efficiently) * data table: max(req/s) * max(ActiveMQ stall time) e.g. to survive 8000 req/s with 60 seconds ActiveMQ stall time, the data table should be >240K in size, so maxdone.scale = 19 (= 512K entries) should be on the safe side also to provide sufficient buffer for temporary load peaks hash table performance ---------------------- Previously, the maximum number of probes to the hash table was set to the hash table size - which resulted in bad insert performance and even worse lookup performance. Now that the hash table is freed of _OPEN records, we can remove this burden and limit the maximum number of probles to a sensible value (10 to start with, configurable as hash_max_probes. As another consequence, as we don't require 100% capacity on the hash table, we don't need to run an exhaustive search upon insert. Thus, probing has been changed from liner to hash (by h2()). only ever insert on ReqStart - and drop if we can't --------------------------------------------------- Keeping up with the VSL is essential. Once we fall behind, we are in real trouble: - If we miss ReqEnd, we will clobber our hash, with drastic effects: - hash lookups become inefficient - inserts become more likely to fail - before we had HASH_Exp (see below), the hash would become useless - When the VSL writer overtakes our reader, we will see corrupt data and miss _many_ VCL Logs and ReqEnds (as many as can be found in the whole VSL), so, again, our hash and data arrays will get clobbered with incomplete data (which needs to be cleaned up by HASH_Exp). The latter point is the most relevant, corrupt records are likely to trigger assertions. Thus, keeping up with the VSL needs to be our primary objective. When the VSL overtakes, we will loose a massive amount auf reconds anyway (and we won't even know how many). As long as we don't stop Varnish when we fall behind, we can't avoid loosing records under certain circumstances anway (for instance, when the backend stalls and the data table runs full), so we should rather drop early, in a controlled manner - and without drastic performance penalty. Under this doctrine, it does not make sense to insert records for VSL_Log or ReqEnd, so if an xid can't be found for these tags, the respective events will get dropped (and logged). performance optimizations ========================= spmcq reader/writer synchronization ----------------------------------- Various measures have been implemented to reduce syscall and general function call overhead for reader/writer synchroniration on the spmcq. Previously, the writer would issue a pthread_cond_signal to potentially wake up a reader, irrespective of whether or not a reader was actually blocking on the CV. - now, the number of waiting readers (workers) is modified inside a lock, but queried first from outside the lock, so if there are no readers waiting the CV is not signalled. - The number of running readers is (attempted to be) kept proportional to the queue length for queue lengths between 0 and 2^qlen_goal.scale to further reduce the number of worker thread block/wakeup transitions under low to averade load. pthread_mutex / ptherad_condvar attributes ------------------------------------------ Attributes are now being used to allow the O/S implementation to choose more efficient low-level synchronization primitives because we know that we are using these only within one multi-threaded process. data table freelist ------------------- To allow for efficient allocation of new data table entries, a free list with local caches is maintained: - The data writer (VSL reader thread) maintains its own freelist and serves requests from it without any synchronization overhead. - Only when the data writer's own freelist is exchausted will it access the global freelist (under a lock). It will take the whole list at once and resume serving new records from its own cache. - Workers also maintain their own freelist of entries to be returned to the global freelist as long as - they are running - there are entries on the global list. Before a worker thread goes to block on the spmcq condvar, it returns all its freelist entries to the global freelist. Also, it will always check if the global list is empty and return any entries immediately if it is. stability improvements ====================== record timeouts --------------- Every hash entry gets added to the insert_list ordered by insertion time. Not any more often then x seconds (currently hard-coded to x=10, check only performed when ReqStart is seen), the list is checked for records which have reached their ttl (configured by hash_ttl, default 120 seconds). These get submitted despite the fact that no ReqEnd has been seen - under the assumption that no ReqEnd is ever to be expected after a certain time has passed. hash evacuation --------------- If no free entry is found when probing all possible locations for an insert, the oldest record is evacuated from the hash and submitted to the backend if its live time has exceeded hash_mlt under the assumption that it is better to submit records early (which are likely to carry useful log information already) than throwing away records. If this behavior is not desired, hash_mtl can be set to hash_ttl. various code changes ==================== * statistics have been reorganized to seperate out - hash - data writer/VSL reader - data reader/worker (partially shared with writer) statistics * print the native thread ID for workers (to allow to correllation with prstat/top output) * workers have a new state when blocking on the spmcq CV: WRK_WAITING / "waiting" in monitor output * because falling behind with VSL reading (the VSL writer overtaking our reader) is so bad, notices are logged whenever the new VSL data pointer is less than the previous one, iow the VSL ring buffer wraps. this is not the same as a detection of the VSL writer overtaking (which would require varnishapi changes), but noting information and some statistics about VSL wraps can (and did) help analyze track down strance issues to VSL overtaking. config file changes =================== * The _scale options maxopen.scale maxdone.scale (new, see below) maxdata.scale are now being used directly, rather than in addition to a base value of 10 as before. 10 is now the minimum value and an EINVAL error will get thrown when lower values are used in the config file. new config options ================== see trackrdrd.h for documentation in comments: * maxdone.scale Scale for records in _DONE states, determines size of - the data table (which is maxopen + maxdone) - the spmcq * qlen_goal.scale Scale for the spmcq queue length goal. All worker threads will be used when the queue length corresponding to the scale is reached. For shorter queue lengths, the number of worker threads will be scaled propotionally. * hash_max_probes Maximum number of probes to the hash. Smaller values increase efficiency, but reduce the capacity of the hash (more ReqStart records may get lost) - and vice versa for higher values. * hash_ttl Maximum time to live for records in the _OPEN state Entries which are older than this ttl _may_ get expired from the trackrdrd state. This should get set to a value significantly longer than your maximum session lifetime in Varnish. * hash_mlt Minimum lifetime for entries in HASH_OPEN before they could get evacuated. Entries are guaranteed to remain in trackrdrd for this duration. Once the mlt is reached, they _may_ get expired when trackrdrd needs space in the hash.
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src/hash.c
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