Red Hat Bugzilla – Bug 249063
readahead does not readahead
Last modified: 2008-06-16 21:55:49 EDT
Description of problem:
Readahead later does not cache
Version-Release number of selected component (if applicable):
Steps to Reproduce:
2. Check the output of free
less then 64M in page cache
A lot more
I downloaded the src rpm and changed the line ~352
readahead(fd, 0, file->size);
while (read(fd, &buff, sizeof(buff)) == sizeof(buff));
Now, I know that this looks ugly, but for some reason readahead function does
not do the trick. When I changed to actual reading it, it work flawlessly.
I have about custom tailored later.sorted, and I do not see a noticable
degradation of performance...
It is about a month and no response yet.
As Fedora community users we also put efforts to make RH products better ones
and in return expect our comments and patches to be reviewed.
Sorry, but the patch is nonsense. The readahead() syscall has to work or has to
be fixed -- any other way is wrong.
I have tried (few months ago) to monitor readahead by bootchart
(http://www.bootchart.org/) and it works.
My plan is to check readahead stuff and generate bootcharts during F8
development -- so, please, be patient :-)
Thanks for your response. If my message has been too harsh please accept my
First I should make some clarifications
-I did not mean that my patch must make its way to the main source
-I specifically said "this hack is ugly" but is a working workaround
-I did not expect (and asked) a working solution on the next day I created the
The bug remained as NEW up until yesterday. What I was unhappy with was, there
was no attention to the bug, nor a comment. (...At least visible to me)
Now, back to the issue:
Please bear with me that I am no kernel expert. But what I understand from
kernel readahead implementation is, it reads n blocks further from the actual
sector on the disk only for the means to push it to the cache. If so, and also
if our requirement is to force fs to read the entire file, do we really need to
rely on readahead in kernel? I know that reading the file has the overhead of
some buffer copying to the userland, but I think the added processing cost there
might be ignored as it does not account much.
Again I do not strictly say, we should *read*, but if kernel readahead is broken
/ likely to be broken in the future just reading the file makes sense.
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