On Mon, Jun 26, 2023 at 03:04:53PM -0300, Marcelo Tosatti wrote:
> Upon closer investigation, it was found that in current codebase, lookup_bh_lru
> is slower than __find_get_block_slow:
>
> 114 ns per __find_get_block
> 68 ns per __find_get_block_slow
>
> So remove the per-CPU buffer_head caching.
>
> Test program:
>
> #define NRLOOPS 200000
> static int __init example_init(void)
> {
> ktime_t s, e;
> s64 delta;
> int i, suc;
>
> bdev = blkdev_get_by_path("/dev/loop0", FMODE_READ, NULL);
> if (IS_ERR(bdev)) {
> printk(KERN_ERR "failed to load /dev/loop0\n");
> return -ENODEV;
> }
>
> suc = 0;
> delta = 0;
> for (i=0; i < NRLOOPS; i++) {
> struct buffer_head *bh;
>
> s = ktime_get();
> bh = __find_get_block(bdev, 1, 512);
> e = ktime_get();
> if (bh) {
> suc++;
> __brelse(bh);
> }
> delta = delta + ktime_to_ns(ktime_sub(e, s));
>
> }
> printk(KERN_ERR "%lld ns per __find_get_block (suc=%d)\n", delta/NRLOOPS, suc);
>
> suc = 0;
> delta = 0;
> for (i=0; i < NRLOOPS; i++) {
> struct buffer_head *bh;
>
> s = ktime_get();
> bh = __find_get_block_slow(bdev, 1);
> e = ktime_get();
> if (bh) {
> suc++;
> __brelse(bh);
> }
> delta = delta + ktime_to_ns(ktime_sub(e, s));
> }
> printk(KERN_ERR "%lld ns per __find_get_block_slow (suc=%d)\n", delta/NRLOOPS, suc);
It occurs to me that this is close to being the best-case scenario for
page-cache lookup as well as for lru lookup. Can you re-run it with
block 4UL * 1024 * 1024 * 1024 instead of block 1?
