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Creating a Shadow L2 Directory

IP.com Disclosure Number: IPCOM000104639D
Original Publication Date: 1993-May-01
Included in the Prior Art Database: 2005-Mar-19
Document File: 4 page(s) / 178K

Publishing Venue

IBM

Related People

Rechtschaffen, R: AUTHOR

Abstract

The combined desiderata of subsetability and servicing a WTWAX L1-CACHE protocol can be combined in a manner which makes the contents of the L1-CACHES a source for L1-misses and extends the size of the L2-CACHE without increasing the capacity of its arrays. However, the directory of the L2 must be extended.

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Creating a Shadow L2 Directory

      The combined desiderata of subsetability and servicing a WTWAX
L1-CACHE protocol can be combined in a manner which makes the
contents of the L1-CACHES a source for L1-misses and extends the size
of the L2-CACHE without increasing the capacity of its arrays.
However, the directory of the L2 must be extended.

A WTWAX cache management protocol is defined as:

o   all stores are written through the L1 cache to the L2 (WT),
o   all lines that are stored into by the processors must be
    allocated (WA - WRITE ALLOCATE), and
o   all lines written into must be held exclusively (X).

      In order to maintain the maximum flexibility of access to the
L1-CACHES within a multiprocessing system and to prevent multiple
copies of the same line to co-reside in the same cache, a means of
determining that a line is in the L1-CACHE, when the congruence class
through which it is accessed does not have the line, is to examine
the L2 directory and determine which congruence class on the given
processor has the line.  The implication of such an algorithm is that
the L2-CACHE has several properties:

o   UNAMBIGUOUS DETERMINATION - The directory of the L2 is organized
    via the Real Congruence Class (RCC).  That is, enough bits from
    the real address of the line are used to specify the congruence

    class within the L2-DIRECTORY so that the determination as to the
    residency of a line in the L1-CACHE will be unambiguous.

o   L1-CACHE POSITION POINTERS - The congruence class of the line in
    the L1-CACHE for each L1 in which it resides will be found
    associated with the L2-DIRECTORY entry.

o   SUBSETABILITY - As a corollary of the previous points the
    contents of the all L1-DIRECTORIES are a subset of the contents
    of the L2-DIRECTORY.

      The algorithm that assures SUBSETABILITY need only be activated
at the point of an L2-CACHE miss.  If following every L2-CACHE miss
the subset property is guaranteed and the L2-CACHE contents change
only at such points, the SUBSETABILITY property is maintained.

L2 SHADOW DIRECTORY- Assume that the size of the L2-DIRECTORY is
twice what is required to handle the number of lines that can reside
in the arrays of the L2-CACHE.  The two issues with regard to
subsetability can be handled by the different type of entries within
the L2-CACHE.

o   For lines that are held with exclusive status, the L2 must have
    the target of the potential stores within its arrays.  Such a
    line is said to be in the L2-REAL DIRECTORY.
o   For lines that are using the L2-DIRECTORY as a means of managing
    collisions for different cache lines that are present in the same
    L1-CACHE, the DIRECTORY function of the L2-DIRECTORY without the
    actual presence of the line within the array can suffice.  These
    lines may be in the L2-SHADOW DIRECTORY.
o   It is also clear that in the main I-LINES do not require th...