Subject: RE: Git 2.54.0-rc1, subtests of t5310, t5326, t5327

From: <[email-masked]>
Date: Wed, 8 Apr 2026 19:15:05 -0400

On April 8, 2026 6:35 PM, Junio C Hamano wrote
>Junio C Hamano <[email-masked]> writes:
>
>> Junio C Hamano <[email-masked]> writes:
>>
>>> To be quite honest, I am not sure if it is even worth using writev()
>>> if we need a loop that protects against shrot writes, so unless I am
>>> grossly mistaken (e.g., perhaps there is some guarantee that there
>>> won't be any short writes for writev() that sends data smaller than
>>> 64k that I missed in the docs), the best course of action might be to
>>> revert the change to use writev() and use the two write(2)s as
>>> before, *if* we actually observe that the current code is broken by
>>> short writes.
>>
>> Ah, sorry, I should have double checked the actual code.  We already
>> use a looping writev_in_full() that wraps writev(), so there is
>> nothing extra that we still need to do to prepare for short writes.
>>
>> Unfortunately, comparing write_in_full() vs writev_in_full(), there is
>> nothing that corresponds to xwrite() that can be used to hide the
>> short writes and chomps an originally larger I/O into smaller pieces.
>
>Oops, the beauty of having xwrite() is *not* that it hides short writes (it
doesn't),
>but it can be used to pretend that short writes happened on platforms with
>unreasonably small I/O limit by setting MAX_IO_SIZE to unusually low.  But
the
>point that ...
>
>> Unlike write() that we may receive a single linear large sequence of
>> bytes, which we can choose to chomp into artificially smaller pieces
>> and write them out (up to 8MB by default), writev() API lets the
>> caller to prepare chunks of memory and I do not think there is a good
>> way for the writev_in_full() at the lower layer to chomp these into
>> smaller pieces, and even if we could, that would defeat the whole
>> reason why we rewrote the original code that used
>> write_in_full() into using writev(), i.e., to avoid extra allocation
>> (and extra system calls---but if your I/O layer is limited to very
>> small writes, no matter how we chop it, you will have to issue extra
>> system calls to flush all of the data out).
>>
>> So, I dunno.
>
>... I doubt that there exists a good way to have xwritev() that wraps
around writev()
>and pretend that a short write happened, instead of issuing a large I/O,
still stands.

I am partial to Peff's runtime approach, personally.

If it helps (probably does not), the limit is due to the DMA/VMM limits on
the
hardware. The NonStop Message system is blindingly fast when sending
messages around between processes on other CPUs - far faster than I have
seen
on most other platforms. But there are physical limits in the chipset. It is
interesting
to use it in an abstracted way, like hacking the TCP/IP stack to pass
handles around.