On 28-Mar-19 8:45 AM, Shahaf Shuler wrote:
Wednesday, March 27, 2019 1:50 PM, Burakov, Anatoly:
Subject: Re: [PATCH] mem: don't use address hint for mapping unless
requested
On 21-Mar-19 8:21 PM, Shahaf Shuler wrote:
patch[1] added an address hint as starting address for 64 bit systems
in case an explicit base virtual address was not set by the user.
The justification for such hint was to help devices that work in VA
mode and has a address range limitation to work smoothly with the eal
memory subsystem.
While the base address value selected may work fine for the eal
initialization, it easily breaks when trying to register external
memory using rte_extmem_register API.
Trying to register anonymous memory on RH x86_64 machine took several
minutes, during them the function eal_get_virtual_area repeatedly
scanned for a good VA candidate.
The attempt to guess which VA address will be free for mapping will
always result in not portable, error prone code:
* different application may use different libraries along w/ DPDK. One
can never guess which library was called first and how much virtual
memory it consumed.
* external memory can be registered at any time in the application run
time.
This patch removes the default address hint and use the address
returned by mmap.
devices with address limitations should suggest to their users a
proper base-virtaddr (EAL arg) to use.
Fixes: 1df21702873d ("mem: use address hint for mapping hugepages")
Cc: sta...@dpdk.org
Cc: alejandro.luc...@netronome.com
[1] commit 1df21702873d ("mem: use address hint for mapping
hugepages")
Signed-off-by: Shahaf Shuler <shah...@mellanox.com>
---
I understand the motivation, however this will lower the reliability of
secondary process initialization.
Can you explain why?
It is very bad if we base the DPDK secondary process design on some predefined
VA address base to be available.
Our whole secondary process is "very bad design" in the first place,
because it relies on address space duplication. This change was
unrelated to it, but it has been shown that setting address hint to this
value (or setting --base-virtaddr to the same address) improves success
rate for secondary process initialization to the point where it doesn't
even register in our internal bug tracker (whereas previously i did
routinely get pings about secondary process init failing).
This is just something we have to live with, and this change has been a
net positive as far as reliability of secondary process initialization goes.
Perhaps, the hinting could be improved by
not trying to use the hint multiple times, but rather just once? I.e.
if using hint succeeded - great, if not - try without one. Most of the time hint
will work correctly, i think.
--
Thanks,
Anatoly
--
Thanks,
Anatoly
