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SubjectRe: sparse file performance (was Re: Is there a "make hole" (truncate in middle) syscall?)

On Fri, 5 Dec 2003, Andy Isaacson wrote:
> I got curious enough to run some tests, and was suprised at the results.
> My machine (Athlon XP 2400+, 2030 MHz, 512 MB, KT400, 2.4.22) can read
> out of buffer cache at 234 MB/s, and off of its IDE disk at 40 MB/s.
> I'd assumed that read(2)ing a holey file would go faster than reading
> out of buffer cache; in theory you could do it completely in L1 cache
> (with a 4KB buffer, it's just a ton of syscalls, some page table
> manipulation, and a bunch of memcpy() out of a single zero page). But
> it turns out that reading a hole is *slower* than reading data from
> buffer cache, just 195 MB/s.

That's because we actually instantiate the page cache pages even for
holes. We have to, or we'd have to special-case them no end (and quite
frankly, "hole read performance" is not something worth special casing,
since it just isn't done under any real load).

So reading a hole implies creating the page cache entry and _clearing_ it.
For each page. So while you may read from the L1, you also have to do
writeback of the _previous_ pages from the L1 into the L2 and eventually
out to memory.

(And eventually the VM also has to get rid of the pages etc, of course).

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