Re: [PATCH 1/2] pipe: change pipe_write() to never add a zero-sized buffer

K Prateek Nayak
2025-02-11T03:59:41Z

Hello Oleg,

On 2/10/2025 10:52 PM, Oleg Nesterov wrote:
> Hi Prateek,
>
> On 02/10, K Prateek Nayak wrote:
>>
>> 1-groups 1.00 [ -0.00]( 7.19) 0.95 [ 4.90](12.39)
>> 2-groups 1.00 [ -0.00]( 3.54) 1.02 [ -1.92]( 6.55)
>> 4-groups 1.00 [ -0.00]( 2.78) 1.01 [ -0.85]( 2.18)
>> 8-groups 1.00 [ -0.00]( 1.04) 0.99 [ 0.63]( 0.77)
>> 16-groups 1.00 [ -0.00]( 1.02) 1.00 [ -0.26]( 0.98)
>>
>> I don't see any regression / improvements from a performance standpoint
>
> Yes, this patch shouldn't make any difference performance-wise, at least
> in this case. Although I was thinking the same thing when I sent "pipe_read:
> don't wake up the writer if the pipe is still full" ;)
>
>> Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
>
> Thanks! Please see v2, I've included you tag.

Thank you. I can confirm it is same as the variant I tested.

>
> Any chance you can also test the patch below?
>
> To me it looks like a cleanup which makes the "merge small writes" logic
> more understandable. And note that "page-align the rest of the writes"
> doesn't work anyway if "total_len & (PAGE_SIZE-1)" can't fit in the last
> buffer.
>
> However, in this particular case with DATASIZE=100 this patch can increase
> the number of copy_page_from_iter()'s in pipe_write(). And with this change
> receiver() can certainly get the short reads, so this can increase the
> number of sys_read() calls.
>
> So I am just curious if this change can cause any noticeable regression on
> your machine.

For the sake of science:

==================================================================
Test : sched-messaging
Units : Normalized time in seconds
Interpretation: Lower is better
Statistic : AMean
==================================================================
Case: baseline[pct imp](CV) merge_writes[pct imp](CV)
1-groups 1.00 [ -0.00](12.39) 1.08 [ -7.62](11.73)
2-groups 1.00 [ -0.00]( 6.55) 0.97 [ 2.52]( 3.01)
4-groups 1.00 [ -0.00]( 2.18) 1.00 [ 0.42]( 1.97)
8-groups 1.00 [ -0.00]( 0.77) 1.03 [ -3.35]( 5.07)
16-groups 1.00 [ -0.00]( 0.98) 1.01 [ -1.37]( 2.20)

I see some improvements up until 4 groups (160 tasks) but beyond that it
goes into a slight regression territory but the variance is large to
draw any conclusions.

Science experiment concluded.

>
> Thank you!
>
> Oleg.
>
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -459,16 +459,16 @@ anon_pipe_write(struct kiocb *iocb, struct iov_iter *from)
> was_empty = pipe_empty(head, pipe->tail);
> chars = total_len & (PAGE_SIZE-1);
> if (chars && !was_empty) {
> - unsigned int mask = pipe->ring_size - 1;
> - struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
> + struct pipe_buffer *buf = pipe_buf(pipe, head - 1);
> int offset = buf->offset + buf->len;
> + int avail = PAGE_SIZE - offset;
>
> - if ((buf->flags & PIPE_BUF_FLAG_CAN_MERGE) &&
> - offset + chars <= PAGE_SIZE) {
> + if (avail && (buf->flags & PIPE_BUF_FLAG_CAN_MERGE)) {
> ret = pipe_buf_confirm(pipe, buf);
> if (ret)
> goto out;
>
> + chars = min_t(ssize_t, chars, avail);
> ret = copy_page_from_iter(buf->page, offset, chars, from);
> if (unlikely(ret < chars)) {
> ret = -EFAULT;
>

--
Thanks and Regards,
Prateek