On Wednesday 20 April 2005 13:52, Linus Torvalds wrote:
> On Wed, 20 Apr 2005, Chris Mason wrote:
> > The patch below with your current tree brings my 100 patch test down to
> > 22 seconds again.
>
> If you ever have a cache_entry bigger than 16384, your code will write
> things out in the wrong order (write the new cache without flushing the
> old buffer).
Whoops
> Finally, if you really want to go fast, you should really try to make your
> writes powers-of-two, ie fill up the buffer entirely rather than saying
> "if I were to overflow, flush it now". It doesn't matter that much for
> some filesystems (especially local and append-only like the patterns are
> here), but it can definitely matter for the stupid ones.
Well, the difference there should be pretty hard to see with any benchmark.
But I was being lazy...new patch attached. This one gets the same perf
numbers, if this is still wrong then I really need some more coffee.
-chris
--- linus.back/read-cache.c 2005-04-20 10:14:23.268310000 -0400
+++ linus/read-cache.c 2005-04-20 14:54:28.554518320 -0400
@@ -232,11 +232,13 @@
SHA_CTX c;
struct cache_header hdr;
int i;
+ #define BUFLEN 16384
+ static char buf[BUFLEN];
+ int len = 0;
hdr.hdr_signature = htonl(CACHE_SIGNATURE);
hdr.hdr_version = htonl(1);
hdr.hdr_entries = htonl(entries);
-
SHA1_Init(&c);
SHA1_Update(&c, &hdr, offsetof(struct cache_header, sha1));
for (i = 0; i < entries; i++) {
@@ -246,13 +248,37 @@
}
SHA1_Final(hdr.sha1, &c);
- if (write(newfd, &hdr, sizeof(hdr)) != sizeof(hdr))
- return -1;
-
+ /* hdr is small right now, but just
+ * in case someone changes that...
+ */
+ if (sizeof(hdr) < BUFLEN) {
+ memcpy(buf, &hdr, sizeof(hdr));
+ len += sizeof(hdr);
+ } else {
+ if (write(newfd, &hdr, sizeof(hdr)) != sizeof(hdr))
+ return -1;
+ }
for (i = 0; i < entries; i++) {
struct cache_entry *ce = cache[i];
int size = ce_size(ce);
- if (write(newfd, ce, size) != size)
+ char *p = (char *)ce;
+ while(size > 0) {
+ int count = size;
+ if (count > BUFLEN - len)
+ count = BUFLEN - len;
+ memcpy(buf + len, p, count);
+ size -= count;
+ len += count;
+ p += count;
+ if (len == BUFLEN) {
+ if (write(newfd, buf, len) != len)
+ return -1;
+ len = 0;
+ }
+ }
+ }
+ if (len) {
+ if (write(newfd, buf, len) != len)
return -1;
}
return 0;
