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edeef03b34
Since reading mutations can consume a large amount of memory, which, moreover, is not predicatable at the time the read is initiated, restrict the number of reads to 100 per shard. This is more than enough to saturate the disk, and hopefully enough to prevent allocation failures. Restriction is applied in column_family::make_sstable_reader(), which is called either on a cache miss or if the cache is disabled. This allows cached reads to proceed without restriction, since their memory usage is supposedly low. Reads from the system keyspace use a separate semaphore, to prevent user reads from blocking system reads. Perhaps we should select the semaphore based on the source of the read rather than the keyspace, but for now using the keyspace is sufficient.