hash: remove the unused m_prefetch_rate_limit

The hash table statistics calculation in BeesHashTable::prefetch_loop
and the data-driven operation of the extent scanner always pulls the
hash table into RAM as fast as the disk will push the data.  We never
use the prefetch rate limit, so remove it.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-hash.cc

@@ -335,9 +335,6 @@ BeesHashTable::fetch_missing_extent(HashType hash)
 	}
 
 	BEESCOUNT(hash_extent_in);
-	// We don't block when fetching an extent but we do slow down the
-	// prefetch thread.
-	m_prefetch_rate_limit.borrow(BLOCK_SIZE_HASHTAB_EXTENT);
 	lock.lock();
 	m_buckets_missing.erase(extent_number);
 }
@@ -596,7 +593,6 @@ BeesHashTable::BeesHashTable(shared_ptr<BeesContext> ctx, string filename, off_t
 	m_writeback_thread("hash_writeback"),
 	m_prefetch_thread("hash_prefetch"),
 	m_flush_rate_limit(BEES_FLUSH_RATE),
-	m_prefetch_rate_limit(BEES_FLUSH_RATE),
 	m_stats_file(m_ctx->home_fd(), "beesstats.txt")
 {
 	// Sanity checks to protect the implementation from its weaknesses

src/bees.h

@@ -444,7 +444,6 @@ private:
 	BeesThread  		m_writeback_thread;
 	BeesThread	        m_prefetch_thread;
 	RateLimiter		m_flush_rate_limit;
-	RateLimiter		m_prefetch_rate_limit;
 	mutex			m_extent_mutex;
 	mutex			m_bucket_mutex;
 	condition_variable	m_condvar;