Messages in this thread | | | Date | Fri, 5 Dec 2003 13:12:21 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: 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).
Linus - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |