Messages in this thread | | | Date | Mon, 8 Dec 2003 14:43:32 -0600 | From | Andy Isaacson <> | Subject | Re: sparse file performance (was Re: Is there a "make hole" (truncate in middle) syscall?) |
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On Fri, Dec 05, 2003 at 01:12:21PM -0800, Linus Torvalds wrote: > 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).
Thanks for the explanation, Linus.
I modified my benchmark to use mmap(2) instead of read(2) and the results are broadly comparable. With a 10MB window, I get 331 MB/s reading out of buffer cache and 185 MB/s reading a hole. Reading a file too large to cache is about the same (disk-limited) speed, 43 MB/s.
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