Message ID | 1363362619-3190-5-git-send-email-pl@kamp.de |
---|---|
State | New |
Headers | show |
On 03/15/2013 09:50 AM, Peter Lieven wrote: > this patch adds the usage of buffer_find_nonzero_offset() > to skip large areas of zeroes. > > compared to loop unrolling presented in an earlier > patch this adds another 50% performance benefit for > skipping large areas of zeroes. loop unrolling alone > added close to 100% speedup. > > Signed-off-by: Peter Lieven <pl@kamp.de> > --- > util/bitops.c | 26 +++++++++++++++++++++++--- > 1 file changed, 23 insertions(+), 3 deletions(-) > + while (size >= BITS_PER_LONG) { > + if ((tmp = *p)) { > + goto found_middle; > + } > + if (((uintptr_t) p) % sizeof(VECTYPE) == 0 > + && size >= BITS_PER_BYTE * sizeof(VECTYPE) > + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) { Another instance where a helper function to check for alignment would be nice. Except this time you have a BITS_PER_BYTE factor, so you would be calling something like buffer_can_use_vectors(buf, size / BITS_PER_BYTE) > + unsigned long tmp2 = > + buffer_find_nonzero_offset(p, ((size / BITS_PER_BYTE) & > + ~(BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * > + sizeof(VECTYPE) - 1))); Type mismatch - buffer_find_nonzero_offset returns size_t, which isn't necessarily the same size as unsigned long. I'm not sure if it can bite you. > + result += tmp2 * BITS_PER_BYTE; > + size -= tmp2 * BITS_PER_BYTE; > + p += tmp2 / sizeof(unsigned long); > + if (!size) { > + return result; > + } > + if (tmp2) { Do you really need this condition, or would it suffice to just 'continue;' the loop? Once buffer_find_nonzero_offset returns anything that leaves size as non-zero, we are guaranteed that the loop will goto found_middle without any further calls to buffer_find_nonzero_offset. > + if ((tmp = *p)) { > + goto found_middle; > + } > + } > } > + p++; > result += BITS_PER_LONG; > size -= BITS_PER_LONG; > } >
Am 19.03.2013 um 17:49 schrieb Eric Blake <eblake@redhat.com>: > On 03/15/2013 09:50 AM, Peter Lieven wrote: >> this patch adds the usage of buffer_find_nonzero_offset() >> to skip large areas of zeroes. >> >> compared to loop unrolling presented in an earlier >> patch this adds another 50% performance benefit for >> skipping large areas of zeroes. loop unrolling alone >> added close to 100% speedup. >> >> Signed-off-by: Peter Lieven <pl@kamp.de> >> --- >> util/bitops.c | 26 +++++++++++++++++++++++--- >> 1 file changed, 23 insertions(+), 3 deletions(-) > >> + while (size >= BITS_PER_LONG) { >> + if ((tmp = *p)) { >> + goto found_middle; >> + } >> + if (((uintptr_t) p) % sizeof(VECTYPE) == 0 >> + && size >= BITS_PER_BYTE * sizeof(VECTYPE) >> + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) { > > Another instance where a helper function to check for alignment would be > nice. Except this time you have a BITS_PER_BYTE factor, so you would be > calling something like buffer_can_use_vectors(buf, size / BITS_PER_BYTE) > >> + unsigned long tmp2 = >> + buffer_find_nonzero_offset(p, ((size / BITS_PER_BYTE) & >> + ~(BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * >> + sizeof(VECTYPE) - 1))); > > Type mismatch - buffer_find_nonzero_offset returns size_t, which isn't > necessarily the same size as unsigned long. I'm not sure if it can bite > you. I will look into it. > >> + result += tmp2 * BITS_PER_BYTE; >> + size -= tmp2 * BITS_PER_BYTE; >> + p += tmp2 / sizeof(unsigned long); >> + if (!size) { >> + return result; >> + } >> + if (tmp2) { > > Do you really need this condition, or would it suffice to just > 'continue;' the loop? Once buffer_find_nonzero_offset returns anything > that leaves size as non-zero, we are guaranteed that the loop will goto > found_middle without any further calls to buffer_find_nonzero_offset. Note in all cases. It will do if the nonzero content is in the first sizeof(unsigned long) bytes. If not, buffer_find_nonzero_offset() is called again. It will return 0 because in the first sizeof(VECTYPE)*BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR bytes is a non-zero byte. To avoid this I placed this check. Peter > >> + if ((tmp = *p)) { >> + goto found_middle; >> + } >> + } >> } >> + p++; >> result += BITS_PER_LONG; >> size -= BITS_PER_LONG; >> } >> > > -- > Eric Blake eblake redhat com +1-919-301-3266 > Libvirt virtualization library http://libvirt.org >
diff --git a/util/bitops.c b/util/bitops.c index e72237a..3c301fa 100644 --- a/util/bitops.c +++ b/util/bitops.c @@ -42,10 +42,30 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size, size -= BITS_PER_LONG; result += BITS_PER_LONG; } - while (size & ~(BITS_PER_LONG-1)) { - if ((tmp = *(p++))) { - goto found_middle; + while (size >= BITS_PER_LONG) { + if ((tmp = *p)) { + goto found_middle; + } + if (((uintptr_t) p) % sizeof(VECTYPE) == 0 + && size >= BITS_PER_BYTE * sizeof(VECTYPE) + * BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) { + unsigned long tmp2 = + buffer_find_nonzero_offset(p, ((size / BITS_PER_BYTE) & + ~(BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR * + sizeof(VECTYPE) - 1))); + result += tmp2 * BITS_PER_BYTE; + size -= tmp2 * BITS_PER_BYTE; + p += tmp2 / sizeof(unsigned long); + if (!size) { + return result; + } + if (tmp2) { + if ((tmp = *p)) { + goto found_middle; + } + } } + p++; result += BITS_PER_LONG; size -= BITS_PER_LONG; }
this patch adds the usage of buffer_find_nonzero_offset() to skip large areas of zeroes. compared to loop unrolling presented in an earlier patch this adds another 50% performance benefit for skipping large areas of zeroes. loop unrolling alone added close to 100% speedup. Signed-off-by: Peter Lieven <pl@kamp.de> --- util/bitops.c | 26 +++++++++++++++++++++++--- 1 file changed, 23 insertions(+), 3 deletions(-)