Disk Cache, Was: O_DIRECT question

[Zan Lynx]

On Sat, 2007-01-13 at 00:03 +0300, Michael Tokarev wrote:
[snip]
> And sure thing, withOUT O_DIRECT, the whole system is almost dead under this
> load - because everything is thrown away from the cache, even caches of /bin
> /usr/bin etc... ;)  (For that, fadvise() seems to help a bit, but not alot).

One thing that I've been using, and seems to work well, is a customized
version of the readahead program several distros use during boot up.

Mine starts off doing:
mlockall(MCL_CURRENT|MCL_FUTURE);
...yadda, yadda...

and for each file listed:
...open, stat stuff...
       if( NULL == mmap(
                NULL, stat_buf.st_size, 
                PROT_READ, MAP_SHARED|MAP_LOCKED|MAP_POPULATE,
                fd, 0)
        ) {
                fprintf(stderr, "'%s' ", file);
                perror("mmap");
        }
...more stuff...
and then ends with:
pause();
and it sits there forever.

As far as I can tell, this makes the program and library code stay in
RAM.  At least, after a drop_caches nautilus doesn't load 12 MB off
disk, it just starts.  It has to be reloaded after software updates and
after prelinking.  I find the 250 MB used to be worthwhile, even if its
kinda Windowsey.

Something like that could keep your system responsive no matter what the
disk cache is doing otherwise.
-- 
Zan Lynx <zlynx@xxxxxxx>

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