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asynchronouslinux-kernelscheduling
itisravi
itisravi

Reputation: 3561

CFQ IO scheduler request queues

The CFQ IO scheduler in Linux has a set of request queues.The synchronous requests from processes go into separate per process request queues while all asynchronous requests go into a set of shared queues.

How are requests classified as synchronous or asynchronous? Does asynchronous in this context mean IO done using kernel AIO? ( and all other normal read()/write() and buffered fread()/fwrite() being counted as synchronous)

Upvotes: 1

Views: 429

Answers (1)

gby
gby

Reputation: 15218

Synchronous requests are those the process is blocked until they complete, asynchronous requests are those that the process can continue in parallel to their completion.

Typically, all normal reads a program makes are synchronous since the process cannot advance until it has the data it requested. Writes however are most often asynchronous by nature - as long as the process is guaranteed to see all writes it has performed, which is taken care by the buffer/page cache, the process does not care when the data is actually written to the storage device once it has called the write system call.

From there on it gets complicated: an fsync() system call is a synchronous request and the same is true for some meta data changing calls on journalled file systems, but not on non journalled ones and so on...

Upvotes: 1

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