Linux Kernel Architecture

Chapter 16: Page and Buffer Cache

window. Mathematically speaking, the maximal readahead size is a fixed point of both functions. In

practical terms, this means that the readahead window can never grow beyond the maximally allowed

value, in this case, 32 pages.

5

10

15

20

25

30

5 10 15 20 25 30

Window size

Request size and last read readahead window, respectively

Initial window size

Next window size

Figure 16-6: How the kernel determines the readahead window depending on the

request size.

Let’s go back toondemand_readahead, which has to set the readahead window with the help of these

auxiliary functions. Three cases are most essential:

1. The current offset is either at the end of the previous readahead window or at the end of the
   interval that was synchronously read in. In both cases, the kernel assumes sequential read
   access, and usesget_next_ra_sizeto compute the new window size as discussed.


2. If the readahead marker was hit, but the previous readahead state does not predict this, then
   most likely two or more concurrent streams perform interleaved reads on the file — and
   invalidate each other’s readahead state in the process. The kernel constructs a new reada-
   head window that suits all readers.


3. If (among others) first read access on a file is performed or a cache miss has happened, a new
   readahead window is set up withget_init_ra_size.

16.5 Implementation of the Buffer Cache

The buffer cache is used not only as an add-on to the page cache but also as an independent cache for

objects that are not handled in pages but in blocks.