Introduction to Cache Memory
Summary
TLDRThis session dives into the concept of cache memory, explaining its importance through the lens of virtual memory and demand paging, using the example of large game files with relatively small main memory requirements. It introduces the three levels of cache memory (L1, L2, L3), their sizes, and speeds, and clarifies terminologies like cache hit, miss, hit latency, and page fault. The session also touches on the principles of spatial and temporal locality that guide data prioritization in cache, setting the stage for further exploration of cache mapping techniques.
Takeaways
- ๐ก The importance of understanding the 'cashier memory' or cache memory is introduced, emphasizing its role in efficient data retrieval.
- ๐ The script explains why not all program code needs to be loaded into main memory by using the analogy of large game files and their relatively small memory requirements.
- ๐ฎ Popular games like GTA 5, Call of Duty, and Hitman 2 are used to illustrate the concept of virtual memory and demand paging.
- ๐ข The script points out the discrepancy between the storage size of games and their main memory requirements, highlighting the efficiency of virtual memory systems.
- ๐ป The concept of cache memory is broken down into different levels, L1, L2, and L3, each with specific roles and capacities within modern computer systems.
- ๐ L1 cache is the smallest and fastest, embedded in the processor, while L2 and L3 caches are larger and used for storing less frequently accessed data.
- ๐ The script explains cache terminology, including 'cache hit' and 'cache miss,' and the processes involved when information is or isn't found in the cache.
- ๐ 'Hit latency' and 'miss latency' are introduced as the time taken for the processor to find or not find information in the cache, respectively.
- ๐ The script touches on the concept of 'page fault' and 'page hit' when information is sought from the main memory or secondary storage.
- ๐ Locality of reference is discussed as the basis for deciding which parts of the main memory should be loaded into the cache, mentioning both spatial and temporal locality.
- ๐ The session concludes with a teaser for upcoming discussions on cache memory mapping techniques and the interaction between cache and main memory.
Q & A
What is the purpose of virtual memory and demand paging in operating systems?
-Virtual memory and demand paging allow the operating system to manage the execution of programs that are larger than the physical memory. They enable programs to be broken into smaller parts, with only the parts currently in use being loaded into the main memory, thus efficiently utilizing the available memory resources.
Why is it not necessary to load the entire code of a game into the main memory while playing?
-The entire code of a game is not required to be loaded into the main memory because of the concept of virtual memory and demand paging. The processor only needs to load the parts of the code that are currently being executed, allowing for efficient use of memory and smooth gameplay.
What are the three levels of cache memory commonly used in modern computer systems?
-The three levels of cache memory commonly used in modern computer systems are L1, L2, and L3 cache. Each level serves a different purpose and has different characteristics in terms of size, speed, and how they are integrated into the system.
How does the L1 cache differ from L2 and L3 caches in terms of integration and speed?
-L1 cache is embedded in the processor itself and is the smallest and fastest among all cache levels. L2 caches, which are also part of the processor but were initially incorporated in the motherboard, are used to store frequently accessed data that cannot fit in L1 due to space limitations. L3 caches are the largest and are shared by all cores of the processor, but they are slower compared to L1 and L2.
What is a cache hit and what is the significance of hit latency?
-A cache hit occurs when the processor successfully finds the required information in the cache. Hit latency is the time taken for this process, and it is an important measure of cache performance. A lower hit latency indicates faster access to the required data.
What happens during a cache miss and what is the associated time period called?
-During a cache miss, the required information is not found in the cache, and the processor seeks it in the next level of memory, which is the main memory. The time taken for this process is called the miss penalty, and it includes the time to fetch the data from the main memory and load it into the cache.
What is the difference between a page fault and a page hit in the context of memory management?
-A page fault occurs when the information sought by the processor is absent from both the cache and the main memory, requiring the operating system to fetch it from secondary storage. A page hit, on the other hand, is when the information is found in the main memory, avoiding the need to access secondary storage.
What is the role of the operating system in managing page faults?
-The operating system manages page faults by looking for the required information in the secondary storage and bringing it back into the main memory. This process is known as page fault service, and the time taken to perform this service is called page fault service time.
What is the concept of locality of reference, and how does it help in cache management?
-Locality of reference is the property of memory access patterns that states recently accessed data is likely to be accessed again in the near future, and data accessed together are likely to be accessed together again. This concept helps in cache management by prioritizing which parts of the main memory should be loaded into the cache based on spatial and temporal locality.
What are spatial and temporal locality, and how do they influence cache replacement policies?
-Spatial locality is the tendency of a program to access nearby memory locations in a short period, while temporal locality is the tendency to access the same memory location multiple times. Cache replacement policies use these concepts to decide which data should be evicted from the cache to make room for new data, aiming to maximize the cache hit rate.
What will be the focus of the next session according to the script?
-The next session will focus on the organization of cache memory, different cache memory mapping techniques, and a detailed understanding of the intercommunication between cache and main memory.
Outlines
๐พ Introduction to Cache Memory and Virtual Memory
This paragraph introduces the concept of cache memory and its importance in computer systems. It explains why not all program code needs to be loaded into main memory by using the example of large video games that require minimal main memory despite their huge storage size, thanks to virtual memory and demand paging. The paragraph also outlines the different levels of cache memory (L1, L2, L3) and their roles in modern computer architectures, emphasizing the multi-core processors and the shared nature of L3 cache among cores.
๐ Understanding Cache Terminologies and Page Faults
The second paragraph delves into cache-related terminologies, explaining what constitutes a cache hit and miss, and the associated latencies. It describes the process that occurs during a cache miss, where the processor seeks information from the main memory and places it in the cache. The concept of page faults and page hits is introduced, detailing the role of the operating system in managing memory hierarchies and the time taken for page fault service. The paragraph concludes with an introduction to the principles of spatial and temporal locality, which guide the prioritization of data in the cache based on the likelihood of future access.
Mindmap
Keywords
๐กCashier Memory
๐กHit Rate or Hit Ratio
๐กVirtual Memory
๐กDemand Paging
๐กL1, L2, and L3 Cache
๐กMulti-core Processors
๐กCache Hit
๐กCache Miss
๐กPage Fault
๐กLocality of Reference
๐กCache Replacement Policies
Highlights
Introduction to the concept of cache memory and its importance in computer systems.
Explanation of why not all program code needs to be in main memory, using the example of a 100-instruction program.
Illustration of the impracticality of loading large game files entirely into main memory, with examples of game storage requirements.
Introduction of virtual memory and demand paging as solutions to efficiently use main memory.
Overview of the recommended main memory requirements for popular games like GTA 5, Call of Duty, and Hitman 2.
Clarification of the multi-level cache memory architecture found in modern computer systems.
Description of L1, L2, and L3 cache levels and their roles in processing.
Explanation of how multi-core processors relate to cache memory levels.
Detailing the size and speed differences among L1, L2, and L3 cache levels.
Introduction to cache-related terminologies such as cache hit, hit latency, and cache miss.
Description of the process that occurs during a cache miss, including seeking information in main memory.
Differentiation between page fault and page hit in the context of memory management.
Introduction to the concept of page fault service time and its significance.
Explanation of how the processor prioritizes which parts of main memory to load into the cache using locality of reference.
Introduction to spatial and temporal locality as approaches for cache management.
Anticipation of future discussions on cache memory mapping techniques and their interaction with main memory.
Conclusion of the session with a summary of the learned concepts and an invitation to the next session.
Transcripts
[Music]
hello everyone
welcome to today's session today we are
going to be properly introduced to the
cashier memory
however before diving straight into it
we will first try to understand the
importance of it
so let's get to learning now during our
previous discussion of the hit rate or
hit ratio
we saw from a program code of 100
instructions 80 were being brought into
the main memory
so you might have thought about why
don't we just bring the entire code of
100 instructions into the memory itself
let me take you through a more realistic
illustration we do use our pcs to play
games
don't we consider the following games
gta 5
call of duty infinite warfare cod modern
warfare the 2019 reboot we are talking
about
hitman 2 the 2018 reboot these are all
great games and the storage requirements
of these
are almost 100 gigabytes the codmw 2019
reboot
even takes up more than 200 gigs of
storage space
however the main memory requirement of
these
are very less compared to this
apparently the recommended
main memory requirement for gta 5 is
only 4 gb
for cod in finite and modern warfare
it's only 8 gb and for hitman 2 it's 16.
now why so it's because our operating
system
provides us with the concept of virtual
memory and demand paging
while playing these or technically
speaking while our processors are
executing the codes of these games
they don't really require the entire 100
gb code at once
and that's the beauty of it that's why
having a way smaller main memory
we can still play the games without
facing any problems whatsoever
and just for the sake of understanding
this concept in simple manner
we took the example of the code segment
of just 100 instructions
now let's talk about the cache again for
the sake of understanding i
mentioned about cache memory as a single
unit in our previous discussions
but to be precise modern day systems
have levels of cache memories
generally there are three levels of
cachet used mostly in today's system
architectures
the l1 l2 and the l3 caching
now as we have multi-core processors in
our machines nowadays
you probably have heard the terms dual
quad
octa-core let me tell you these belong
to the mimd family of flint's taxonomy
i hope you remember that from our
computer architecture classifications
discussion
now l1 cache is embedded in the
processor itself
right from the days of its origin later
when the l2 caches emerged
they were incorporated in the
motherboard in their initial days
but now they are also a part of the
processor itself
to be precise different cores of the
processor have their own
l1 and l2 caches now coming to the l3
caches they are also implanted in the
processor
yet shared by all the different cores of
the processor
by now you probably have the idea about
these levels
size wise l1 is the smallest yet it is
the fastest among
all the other caches l2 caches come
after
l1 and these are used to store the
frequently accessed data
which are second in priority also
frankly can't really be incorporated
within the l1 cache
due to the limitation of space finally
the l3 caches are the largest of all
and these are also called shared cache
now i hope the idea of different cache
levels are clear to you
in our later discussions for the sake of
simplicity
we will assume to have only a single
cache mostly
however i'll provide detailed
illustration of the various levels
during the explanations of cache level
related numerical problems
now let's get to know about a few
terminologies related to cache
the first one cache hit during execution
if the processor is able to find out the
required information
in the cachet we call it a cache ahead
and the time required by this process is
known as hit latency
here using a specific data structure
called tag directory
the processor finds out whether the
required information
is present in the cache or not now if
the information is absent from the
cachet that is if the info
is missing from the cachet we call it
cache a miss
in this case as discussed earlier the
processor will seek for the information
in the next level of memory
which is the main memory and bring it
from there also
meanwhile place it in the cache itself
this entire
period of time is called
an fyi if the information is absent
also from the main memory the situation
is called
page fault and if found we call it page
hit
during page fault the os as it manages
all the intercommunication between the
main memory and secondary memory
looks for the information in the last
level of the hierarchy that is the
secondary storage
and brings it back in the main memory
this entire process is known as
page fault service thus the time taken
to perform it
is termed as page fault service time now
we
already know that the information whose
requirement frequency is very much
higher than the others
are generally kept in the cache now this
prioritizing of the parts of the main
memory which are to be loaded inside the
cache
is done using the locality of reference
in simpler words
there are two approaches based on which
the processor can decide
which data of the main memory should be
placed inside the cache
the first approach is based on spatial
locality
it means at a particular point of time
if a memory location
is referred by the processor chances are
the nearby locations will probably be
referred in near future
next approach is based on temporal
locality
it means if the memory location is
referred then there are chances that it
will be referred again
this idea will be more clear during the
study of
cache replacement policies well that was
all for this session
i guess since we learned the
organization of cache memory
now it will be easier for us to get into
the different cache memory mapping
techniques
and we can have a better understanding
of the intercommunication of cachet
and main memory in details hope to see
you in the next one
thank you all for watching
[Music]
you
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