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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 583
552
MULTIPLE PROCESSOR SYSTEMS
CHAP. 8
packet to it, not only will the incoming packet be lost, but also a page of innocent
memory will be ruined, probably with disastrous consequences shortly.
These problems can be avoided by having system calls to pin and unpin pages
in memory, marking them as temporarily unpageable. However, having to make a
system call to pin the page containing each outgoing packet and then having to
make another call later to unpin it is expensive.
If packets are small, say, 64 bytes
or less, the overhead for pinning and unpinning every buffer is prohibitive. For
large packets, say, 1 KB or more, it may be tolerable.
For sizes in between, it de-
pends on the details of the hardware. Besides introducing a performance hit, pin-
ning and unpinning pages adds to the software complexity.
Remote Direct Memory Access
In some fields, high network latencies are simply not acceptable. For instance,
for certain applications in high-performance computing the computation time is
strongly dependent on the network latency. Likewise, high-frequency trading is all
about having computers perform transactions (buying and selling stock) at ex-
tremely high speeds—every microsecond counts. Whether or not it is wise to have
computer programs trade millions of dollars worth of stock in a millisecond, when
pretty much all software tends to be buggy, is an interesting question for dining
philosophers to consider when they are not busy grabbing their forks. But not for
this book. The point here is that if you manage to get the latency down, it is sure to
make you very popular with your boss.
In these scenarios, it pays to reduce the amount of copying. For this reason,
some network interfaces support
RDMA
(
Remote Direct Memory Access
), a
technique that allows one machine to perform a direct memory access from one
computer to that of another. The RDMA does not involve either of the operating
system and the data is directly fetched from, or written to, application memory.
RDMA sounds great, but it is not without its disadvantages. Just like normal
DMA, the operating system on the communicating nodes must pin the pages invol-
ved in the data exchange. Also, just placing data in a remote computer’s memory
will not reduce the latency much if the other program is not aware of it. A suc-
cessful RDMA does not automatically come with an explicit notification. Instead, a
common solution is that a receiver polls on a byte in memory. When the transfer is
done, the sender modifies the byte to signal the receiver that there is new data.
While this solution works, it is not ideal and wastes CPU cycles.
For really serious high-frequency trading, the network cards are custom built
using field-programmable gate arrays. They have wire-to-wire latency, from re-
ceiving the bits on the network card to transmitting a message to buy a few million
worth of something, in well under a microsecond.
Buying $1 million worth of
stock in 1
μ
sec gives a performance of 1 terabuck/sec, which is nice if you can get
the ups and downs right, but is not for the faint of heart.
Operating systems do not
play much of a role in such extreme settings.
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