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So far, we have looked at relatively small components of the SGA. Now we are going to look at one that is potentially huge in size. The block buffer cache is where Oracle stores database blocks before writing them to disk and after reading them in from disk. This is a crucial area of the SGA for us. Make it too small and our queries will take forever to run. Make it too big and we'll starve other processes (e.g., we won't leave enough room for a dedicated server to create its PGA, and we won't even get started). In earlier releases of Oracle, there was a single block buffer cache, and all blocks from any segment went into this single area. Starting with Oracle 8.0, we had three places to store cached blocks from individual segments in the SGA: Default pool: The location where all segment blocks are normally cached. This is the original and, previously, the only buffer pool. Keep pool: An alternate buffer pool where by convention you assign segments that are accessed fairly frequently, but still get aged out of the default buffer pool due to other segments needing space. Recycle pool: An alternate buffer pool where by convention you assign large segments that you access very randomly, and which would therefore cause excessive buffer flushing of many blocks from many segments. There s no benefit to caching such segments because by the time you wanted the block again, it would have been aged out of the cache. You would separate these segments out

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This basic implementation Parallel first converts the input task sequence to an array and then creates mutable state count and results to record the progress of the parallel computations. It then iterates through the tasks and queues each for execution in the .NET thread pool. Upon completion, each writes its result and decrements the counter using an atomic Interlocked. Decrement operator, discussed further in the section Understanding Shared-Memory Concurrency at the end of this chapter. The last process to finish calls the continuation with the collected results. In practice, Parallel is implemented slightly differently to take into account exceptions and cancellation; once again, see the F# library code for full details.

from the segments in the default and keep pools so they would not cause those blocks to age out of the cache. Note that in the keep and recycle pool descriptions I used the phrase "by convention." There is nothing in place to ensure that you use either the keep pool or the recycle pool in the fashion described. In fact, the three pools manage blocks in a mostly identical fashion; they do not have radically different algorithms for aging or caching blocks. The goal here was to give the DBA the ability to segregate segments to hot, warm, and do not care to cache areas. The theory was that objects in the default pool would be hot enough (i.e., used enough) to warrant staying in the cache all by themselves. The cache would keep them in memory since they were very popular blocks. If you had some segments that were fairly popular but not really hot, these would be considered the warm blocks. These segments blocks could get flushed from the cache to make room for blocks you used infrequently (the do not care to cache blocks). To keep these warm segments' blocks cached, you could do one of the following: Assign these segments to the keep pool, in an attempt to let the warm blocks stay in the buffer cache longer. Assign the do not care to cache segments to the recycle pool, keeping the recycle pool fairly small so as to let the blocks come into the cache and leave the cache rapidly (decrease the overhead of managing them all).

Having to do one of these two things increased the management work the DBA had to perform, as there were three caches to think about, size, and assign objects to. Remember also that there is no sharing among them, so if the keep pool has lots of unused space, it won't give it to the overworked default or recycle pool. All in all, these pools were generally regarded as a very fine, low-level tuning device, only to be used after most other tuning alternatives had been looked at (if I could rewrite a query to do one-tenth the I/O rather then set up multiple buffer pools, that would be my choice!). Starting in Oracle9i, the DBA had up to four more optional caches, the db_Nk_caches, to consider in addition to the default, keep, and recycle pools. These caches were added in support of multiple blocksizes in the database. Prior to Oracle9i, a database would have a single blocksize (typically 2KB, 4KB, 8KB, 16KB, or 32KB). Starting with Oracle9i, however, a database can have a default blocksize, which is the size of the blocks stored in the default, keep, or recycle pool, as well as up to four nondefault blocksizes, as explained in 3 Files . The blocks in these buffer caches are managed in the same way as the blocks in the original default pool there are no special algorithm changes for them either. Let's now move on to see how the blocks are managed in these pools.