Discussion questions
- What is the basic idea of distributed shared memory?
- How does distributed shared memory compare to virtual memory on a single CPU system? How about a system with many cores?
- How aware does a programmer need to be of DSM to use it? How aware to use it efficiently?
- What are the key mechanisms supporting DSM?
- How common do you think is DSM today in the cloud today? Why?
Notes
Lecture 5
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Group reports
- so far they look mostly good
- please try to add some structure to it
- organizing around questions asked/topics discussed helps
- section headings are nice
- 1-2 pages is what most of you are turning in, which is good
- but please use complete sentences, don't just use bullet points/phrases. It
is just harder to understand/more ambiguous
DSM
What is the basic idea of distributed shared memory?
- a process should be able to run across multiple computers
- different threads on different hosts
- but all sharing the same memory/address space
- instead of multiple processes communicating over the network,
we have one process sharing info with itself through shared memory,
just like any multithreaded program
- (we can just share part of a process's address space, then it is just
like two processes sharing part of their memory on a single system)
Do we like to program multithreaded programs?
- in general, it is the hardest way to implement things
- but on multicore systems, can be the fastest
- because shared memory is a fast way to share state
- avoid copies of data from messages
Does this apply to a cluster of systems?
- NO, not at all
- because "shared memory" is an illusion implemented by COPYING DATA OVER THE NETWORK
- so we can never be faster than just sending messages back and forth
- so, why do DSM if not performance?
- ease of use, or
- legacy code
How does distributed shared memory compare to virtual memory on a single CPU system?
- very similar
- basically we're swapping across the network instead of to disk
- but data can change when it is swapped out
How about a system with many cores?
- very similar, except...
- everything implemented in hardware
- VERY fast network, ultra low latency
- don't have to worry about network failures
- system is dead then
- copying has to happen all the time
- between caches of cores and main memory
How common do you think is DSM today in the cloud today? Why?
- In a sense, DSM is alive and well, just on multicore systems
- but everywhere else...not so much
- just isn't more efficient in software
- better to just send messages using some other abstraction
Isn't a distributed cache a DSM?
- not really, it is much more specialized
- think content distribution networks (CDNs)
How aware does a programmer need to be of DSM to use it?
- not at all, it is transparent
How aware to use it efficiently?
- VERY aware, if you aren't careful performance will go into the toliet
- and it is hard to tell you're making things slow, we don't think of memory access that way most of the time
- think about how difficult it is to make cache-efficient code
- add in a network and complexity goes up, as does cost of failures to do
the right thing
Eventual consistency vs strong consistency
- if two nodes access the same memory, do they HAVE to see the same thing immediately?
- if they don't, you can get away with eventual consistency and improve performance
- but then, why use shared memory?
If you're tuning your system, is it easier to
- play with memory placement, DSM algorithms, or
- optimize network usage?
Note the trend here
- when we try to fool the developer that there isn't a network, we have
performance & scaling bottlenecks
- need abstractions that are inherently network-aware
- account for latency, bandwidth, reliability, (& security issues)