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instruction cache or data cache

Discussion in 'Embedded' started by karthikbalaguru, Oct 17, 2007.

  1. Hi,

    Should my processor have instruction cache more ?
    or
    Should my processor have data cache more ?

    Which should be more for an efficient processor ?

    Thx in advans,
    Karthik Balaguru
     
    karthikbalaguru, Oct 17, 2007
    #1
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  2. karthikbalaguru

    Joe Peric Guest

    More of both, If you can ;)

    It really depends on the ISA and the workloads your expecting, as well
    as the rest of the architecture surrounding the chip. You should paint
    us a picture of what you already have and then you might get some
    better help. :)

    Joe
     
    Joe Peric, Oct 17, 2007
    #2
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  3. I am designing a system that should do efficient processing of data
    received and
    also communication with a FPGA(Shared Memory) that will have data
    written/read by all other processors (2 other DSP processors). All
    these processors can communicate the data/info among themselves via
    the FPGA(Shared Memory) and can perform algorithmic manipulation of
    those data after reading from FPGA or before writing to the FPGA.There
    is one more controller that takes of the auxillary devices and that
    will also be communicating with the FPGA for retreiving the data for
    its auxillary devices.

    Karthik Balaguru
     
    karthikbalaguru, Oct 17, 2007
    #3
  4. karthikbalaguru

    jetmarc Guest

    I am designing a system that should do efficient processing of data

    It all depends on what kind of processing is to be done.

    If the algorithms require touching the data just once (like
    compressing, converting or encrypting a stream), a data-cache won't
    help much. It will be merely reduced to "prefetch" and "writebuffer"
    functionality. It might even hurt performance, especially when you
    need each data word just once, and in random order. However, if you
    need to touch each data word several times (but can't keep them in CPU
    registers in the meantime), then a correctly sized data-cache will
    help a lot.

    If the algorithm code is very repetitive (tight loops et al), then an
    instruction cache will help too. On the other hand, if you have to
    branch to lots of possible execution flows, then the instruction cache
    won't be very useful. It too can hurt in some situations.

    So unless you specify your situation with more detail, our answers
    can't be more specific either.

    Regards,
    Marc
     
    jetmarc, Oct 17, 2007
    #4
  5. The processor is yours, so it is for you to decide.
    None. Ideally, the bus should run at full speed.


    Vladimir Vassilevsky
    DSP and Mixed Signal Design Consultant
    http://www.abvolt.com
     
    Vladimir Vassilevsky, Oct 17, 2007
    #5
  6. karthikbalaguru

    MitchAlsup Guest

    This depends upon the target of the design point. If the design is
    shooting at less than 1/2 of the maximum performance if all stops are
    taken out, then you can get away with a unified cache, expecially if
    wide access is allowed to the cache (like 128-bits/cycle). This kind
    of width allows several instruction accesses to take only one cycle,
    leaving the other 3/4 of the cycles for data accesses. I once designed
    a pipeline around this very notion, and found that it works quite well
    (at the 1/2 less than all out max performance).

    A unified cache may actually have more cache and a better range of
    ballance points between I and D stream accesses. Separate I and D
    caches are for when one cannot solve the interference problems (which
    typically acrue with targets of max performance possible--"damn the
    torpedos, full speed ahead".

    useful additions:

    If the instruction set is byte aligned, then consider making the basic
    cache access byte aligned for both instruction fetches and data
    fetches. This is a straight tradeoff between power and utility, and
    when making a unified cache it definately helps lower the interference
    between the I and D streams.

    This kind of cache also gets rid of Self-modifying code issues as the
    modified data is in the same cache as the instructions. However, it
    does NOT get rid of cross modifying code issues.
     
    MitchAlsup, Oct 17, 2007
    #6
  7. karthikbalaguru

    MitchAlsup Guest

    If your target performance is less than 1/2 of the max performance
    achievable with all the tricks of the trade, then you can get by with
    a unified cache. This is especially true if you fetch wide items from
    the cache (128-bits) and double especially true if you can fetch a
    misaligned 128-bit item from the cache. These two things get the
    natural instructiion fetch out of the way from the data fetches for
    most applications.
     
    MitchAlsup, Oct 18, 2007
    #7
  8. What am I missing here? If you can tolerate 1/2 the max performance,
    why are you concerned with any cache?
     
    Everett M. Greene, Oct 18, 2007
    #8
  9. karthikbalaguru

    MitchAlsup Guest

    Non-cache memory is so slow that one cannot obtain even 1/10 max
    performance without caching. So caches have to be somewhere.

    What I am suggesting is that between 0.X and 1.0 of "as high a
    performance as you can architect" one pretty much needs a harvard
    cache design, because the interference between the I and D streams is
    (for practical purposes) unsolvable for a unified cache.

    X can be somewhere in the 0.4 to 0.6 range of performance where a top-
    end Intel or AMD CPU is rated at 1.0.

    Whereas under 0.X of max performance, the unified cache comes back
    into vouge and will end up out performing the separate I&D caches of
    equivalent storage space.

    Mitch
     
    MitchAlsup, Oct 19, 2007
    #9
  10. In general, instruction caches tend to have higher hit rate the data
    caches. Data caches have to deal with less predictable data reads and
    writes. Also, data cache entries need to be written back to the main
    memory.
     
    EventHelix.com, Oct 20, 2007
    #10
  11. karthikbalaguru

    Chris Maryan Guest

    You could simulate for your particular application. Write a simple
    simulator that's behaviourally close to your intended design and run
    something close to your intended application on it. You can
    parametrize the cache size (and associativity, etc.) and then try
    different sizes to see whats best.

    Chris
     
    Chris Maryan, Oct 20, 2007
    #11
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