On 2019/11/12 下午5:12, Miklos Szeredi wrote: > On Tue, Nov 12, 2019 at 8:53 AM Jiufei Xue <jiufei.xue@xxxxxxxxxxxxxxxxx> wrote: >> >> Hi Miklos, >> >> A performance regression is observed since linux v4.19 when we do aio >> test using fio with iodepth 128 on overlayfs. And we found that queue >> depth of the device is always 1 which is unexpected. >> >> After investigation, it is found that commit 16914e6fc7 >> (“ovl: add ovl_read_iter()”) and commit 2a92e07edc >> (“ovl: add ovl_write_iter()”) use do_iter_readv_writev() to submit >> requests to real filesystem. Async IOs are converted to sync IOs here >> and cause performance regression. >> >> I wondered that is this a design flaw or supposed to be. > > It's not theoretically difficult to fix. The challenge is to do it > without too much complexity or code duplication. > > Maybe best would be to introduce VFS helpers specially for stacked > operation such as: > > ssize_t vfs_read_iter_on_file(struct file *file, struct kiocb > *orig_iocb, struct iov_iter *iter); > ssize_t vfs_write_iter_on_file(struct file *file, struct kiocb > *orig_iocb, struct iov_iter *iter); > > Implementation-wise I'm quite sure we need to allocate a new kiocb and > initialize it from the old one, adding our own completion callback. > Yes, I totally agree with you. I will try to fix this regression and send the patch. Thanks, Jiufei > Thanks, > Miklos >
