Question

what are average read cycle time and write cycle time in write through and write back cache?Elaborate the concept please.

Comments

Here is correct answer given by Arjun 

https://gateoverflow.in/14480/formula-write-back-write-through-access-time-parallel-serial

Answer 1

WRITE THROUGH POLICY:
CACHE and MAIN MEMORY is simultaneously updated.it eliminates the problem of CACHE coherence in SINGLE PROCESSOR SYSTEM.
here updation time will be Max(Cache time,memory time) which is memory time and is feasible only for less updations.
AVERAGE READ TIME=(hit ratio of read)*cache time+(1-HIT ratio read)(cache time + memory time)
AVERAGE WRITE TIME= (hit ratio of write)*(UPDATION TIME)+(1-hit ratio write) (updation time + memory time)

WRITE BACK POLICY:
whenever a block is choosen for replacement at that time main memory is updated.
here updation time depends on dirty bit concept
if dirty bit is 1 it means block in cache is modified so here block time (for incoming block) +( block time for updating block) + cache time
if dirty bit is 0 it means no modification is there  so we dnt need to update in that case block time for (incoming block) + cache time
this is how updation time is caclculated based on the dirty bit.

Comments

" we are counting memory access time twice" --- But one for accessing a block of word and one for accessing a single word

One for accessing a cache block from main memory and another for accessing the word from cache? That is correct, assuming a hierarchical memory. On modern computers (simultaneous access), along with the cache block fetch from main memory, the requested word is forwarded to the CPU in parallel. So, time would just be the main memory access time. But for GATE, we should see how the question is asked and answer accordingly. 

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@arjun Sir please clear the concept. still ambiguous after reading the comment.

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@arpit 

read here

1.  https://gateoverflow.in/14480/formula-write-back-write-through-access-time-parallel-serial
2.  https://gateoverflow.in/28959/write-through-cache-policy-questions-per-answer-should-given?