Hi Song,
On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
... ...
> The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> direct memory mapping fragmentation. This leads to non-trivial performance
> improvements.
>
> For our web service production benchmark, bpf_prog_pack on 4kB pages
> gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> believe this is also significant for other companies with many thousand
> servers.
>
I'm evaluationg performance impact due to direct memory mapping
fragmentation and seeing the above, I wonder: is the performance improve
mostly due to prog pack and hugepage instead of less direct mapping
fragmentation?
I can understand that when progs are packed together, iTLB miss rate will
be reduced and thus, performance can be improved. But I don't see
immediately how direct mapping fragmentation can impact performance since
the bpf code are running from the module alias addresses, not the direct
mapping addresses IIUC?
I appreciate it if you can shed some light on performance impact direct
mapping fragmentation can cause, thanks.
Hi Aaron,
On Mon, Jun 20, 2022 at 4:12 AM Aaron Lu <[email protected]> wrote:
>
> Hi Song,
>
> On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
>
> ... ...
>
> > The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> > direct memory mapping fragmentation. This leads to non-trivial performance
> > improvements.
> >
> > For our web service production benchmark, bpf_prog_pack on 4kB pages
> > gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> > bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> > bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> > believe this is also significant for other companies with many thousand
> > servers.
> >
>
> I'm evaluationg performance impact due to direct memory mapping
> fragmentation and seeing the above, I wonder: is the performance improve
> mostly due to prog pack and hugepage instead of less direct mapping
> fragmentation?
>
> I can understand that when progs are packed together, iTLB miss rate will
> be reduced and thus, performance can be improved. But I don't see
> immediately how direct mapping fragmentation can impact performance since
> the bpf code are running from the module alias addresses, not the direct
> mapping addresses IIUC?
You are right that BPF code runs from module alias addresses. However, to
protect text from overwrites, we use set_memory_x() and set_memory_ro()
for the BPF code. These two functions will set permissions for all aliases
of the memory, including the direct map, and thus cause fragmentation of
the direct map. Does this make sense?
Thanks,
Song
On Mon, Jun 20, 2022 at 07:11:45PM +0800, Aaron Lu wrote:
> Hi Song,
>
> On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
>
> ... ...
>
> > The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> > direct memory mapping fragmentation. This leads to non-trivial performance
> > improvements.
> >
> > For our web service production benchmark, bpf_prog_pack on 4kB pages
> > gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> > bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> > bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> > believe this is also significant for other companies with many thousand
> > servers.
> >
>
> I'm evaluationg performance impact due to direct memory mapping
> fragmentation
BTW how exactly are you doing this?
Luis
> and seeing the above, I wonder: is the performance improve
> mostly due to prog pack and hugepage instead of less direct mapping
> fragmentation?
>
> I can understand that when progs are packed together, iTLB miss rate will
> be reduced and thus, performance can be improved. But I don't see
> immediately how direct mapping fragmentation can impact performance since
> the bpf code are running from the module alias addresses, not the direct
> mapping addresses IIUC?
>
> I appreciate it if you can shed some light on performance impact direct
> mapping fragmentation can cause, thanks.
On Mon, Jun 20, 2022 at 09:03:52AM -0700, Song Liu wrote:
> Hi Aaron,
>
> On Mon, Jun 20, 2022 at 4:12 AM Aaron Lu <[email protected]> wrote:
> >
> > Hi Song,
> >
> > On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
> >
> > ... ...
> >
> > > The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> > > direct memory mapping fragmentation. This leads to non-trivial performance
> > > improvements.
> > >
> > > For our web service production benchmark, bpf_prog_pack on 4kB pages
> > > gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> > > bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> > > bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> > > believe this is also significant for other companies with many thousand
> > > servers.
> > >
> >
> > I'm evaluationg performance impact due to direct memory mapping
> > fragmentation and seeing the above, I wonder: is the performance improve
> > mostly due to prog pack and hugepage instead of less direct mapping
> > fragmentation?
> >
> > I can understand that when progs are packed together, iTLB miss rate will
> > be reduced and thus, performance can be improved. But I don't see
> > immediately how direct mapping fragmentation can impact performance since
> > the bpf code are running from the module alias addresses, not the direct
> > mapping addresses IIUC?
>
> You are right that BPF code runs from module alias addresses. However, to
> protect text from overwrites, we use set_memory_x() and set_memory_ro()
> for the BPF code. These two functions will set permissions for all aliases
> of the memory, including the direct map, and thus cause fragmentation of
> the direct map. Does this make sense?
Guess I didn't make it clear.
I understand that set_memory_XXX() will cause direct mapping split and
thus, fragmented. What is not clear to me is, how much impact does
direct mapping fragmentation have on performance, in your case and in
general?
In your case, I guess the performance gain is due to code gets packed
together and iTLB gets reduced. When code are a lot, packing them
together as a hugepage is a further gain. In the meantime, direct
mapping split (or not) seems to be a side effect of this packing, but it
doesn't have a direct impact on performance.
One thing I can imagine is, when an area of direct mapping gets splited
due to permission reason, when that reason is gone(like module unload
or bpf code unload), those areas will remain fragmented and that can
cause later operations that touch these same areas using more dTLBs
and that can be bad for performance, but it's hard to say how much
impact this can cause though.
Regards,
Aaron
On Mon, Jun 20, 2022 at 11:31:39AM -0700, Luis Chamberlain wrote:
> On Mon, Jun 20, 2022 at 07:11:45PM +0800, Aaron Lu wrote:
> > Hi Song,
> >
> > On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
> >
> > ... ...
> >
> > > The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> > > direct memory mapping fragmentation. This leads to non-trivial performance
> > > improvements.
> > >
> > > For our web service production benchmark, bpf_prog_pack on 4kB pages
> > > gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> > > bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> > > bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> > > believe this is also significant for other companies with many thousand
> > > servers.
> > >
> >
> > I'm evaluationg performance impact due to direct memory mapping
> > fragmentation
>
> BTW how exactly are you doing this?
Right now I'm mostly collecting materials from the web :-)
Zhengjun has run some extensive microbenmarks with different page size
for direct mapping and on different server machines a while ago, here
is his report:
https://lore.kernel.org/linux-mm/[email protected]/
Quoting part of the conclusion:
"
This leads us to conclude that although 1G mappings are a
good default choice, there is no compelling evidence that it must be the
only choice, or that folks deriving benefits (like hardening) from
smaller mapping sizes should avoid the smaller mapping sizes.
"
I searched the archive and found there is performance problem when
kernel text huge mapping gets splitted:
https://lore.kernel.org/lkml/[email protected]/
But I haven't found a report complaining direct mapping fragmentation yet.
On Mon, Jun 20, 2022 at 6:32 PM Aaron Lu <[email protected]> wrote:
>
> On Mon, Jun 20, 2022 at 09:03:52AM -0700, Song Liu wrote:
> > Hi Aaron,
> >
> > On Mon, Jun 20, 2022 at 4:12 AM Aaron Lu <[email protected]> wrote:
> > >
> > > Hi Song,
> > >
> > > On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
> > >
> > > ... ...
> > >
> > > > The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> > > > direct memory mapping fragmentation. This leads to non-trivial performance
> > > > improvements.
> > > >
> > > > For our web service production benchmark, bpf_prog_pack on 4kB pages
> > > > gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> > > > bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> > > > bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> > > > believe this is also significant for other companies with many thousand
> > > > servers.
> > > >
> > >
> > > I'm evaluationg performance impact due to direct memory mapping
> > > fragmentation and seeing the above, I wonder: is the performance improve
> > > mostly due to prog pack and hugepage instead of less direct mapping
> > > fragmentation?
> > >
> > > I can understand that when progs are packed together, iTLB miss rate will
> > > be reduced and thus, performance can be improved. But I don't see
> > > immediately how direct mapping fragmentation can impact performance since
> > > the bpf code are running from the module alias addresses, not the direct
> > > mapping addresses IIUC?
> >
> > You are right that BPF code runs from module alias addresses. However, to
> > protect text from overwrites, we use set_memory_x() and set_memory_ro()
> > for the BPF code. These two functions will set permissions for all aliases
> > of the memory, including the direct map, and thus cause fragmentation of
> > the direct map. Does this make sense?
>
> Guess I didn't make it clear.
>
> I understand that set_memory_XXX() will cause direct mapping split and
> thus, fragmented. What is not clear to me is, how much impact does
> direct mapping fragmentation have on performance, in your case and in
> general?
>
> In your case, I guess the performance gain is due to code gets packed
> together and iTLB gets reduced. When code are a lot, packing them
> together as a hugepage is a further gain. In the meantime, direct
> mapping split (or not) seems to be a side effect of this packing, but it
> doesn't have a direct impact on performance.
>
> One thing I can imagine is, when an area of direct mapping gets splited
> due to permission reason, when that reason is gone(like module unload
> or bpf code unload), those areas will remain fragmented and that can
> cause later operations that touch these same areas using more dTLBs
> and that can be bad for performance, but it's hard to say how much
> impact this can cause though.
Yes, we have data showing the direct mapping remaining fragmented
can cause non-trivial performance degradation. For our web workload,
the difference is in the order of 1%.
Thanks,
Song
On Mon, Jun 20, 2022 at 07:51:24PM -0700, Song Liu wrote:
> On Mon, Jun 20, 2022 at 6:32 PM Aaron Lu <[email protected]> wrote:
> >
> > On Mon, Jun 20, 2022 at 09:03:52AM -0700, Song Liu wrote:
> > > Hi Aaron,
> > >
> > > On Mon, Jun 20, 2022 at 4:12 AM Aaron Lu <[email protected]> wrote:
> > > >
> > > > Hi Song,
> > > >
> > > > On Fri, May 20, 2022 at 04:57:50PM -0700, Song Liu wrote:
> > > >
> > > > ... ...
> > > >
> > > > > The primary goal of bpf_prog_pack is to reduce iTLB miss rate and reduce
> > > > > direct memory mapping fragmentation. This leads to non-trivial performance
> > > > > improvements.
> > > > >
> > > > > For our web service production benchmark, bpf_prog_pack on 4kB pages
> > > > > gives 0.5% to 0.7% more throughput than not using bpf_prog_pack.
> > > > > bpf_prog_pack on 2MB pages 0.6% to 0.9% more throughput than not using
> > > > > bpf_prog_pack. Note that 0.5% is a huge improvement for our fleet. I
> > > > > believe this is also significant for other companies with many thousand
> > > > > servers.
> > > > >
> > > >
> > > > I'm evaluationg performance impact due to direct memory mapping
> > > > fragmentation and seeing the above, I wonder: is the performance improve
> > > > mostly due to prog pack and hugepage instead of less direct mapping
> > > > fragmentation?
> > > >
> > > > I can understand that when progs are packed together, iTLB miss rate will
> > > > be reduced and thus, performance can be improved. But I don't see
> > > > immediately how direct mapping fragmentation can impact performance since
> > > > the bpf code are running from the module alias addresses, not the direct
> > > > mapping addresses IIUC?
> > >
> > > You are right that BPF code runs from module alias addresses. However, to
> > > protect text from overwrites, we use set_memory_x() and set_memory_ro()
> > > for the BPF code. These two functions will set permissions for all aliases
> > > of the memory, including the direct map, and thus cause fragmentation of
> > > the direct map. Does this make sense?
> >
> > Guess I didn't make it clear.
> >
> > I understand that set_memory_XXX() will cause direct mapping split and
> > thus, fragmented. What is not clear to me is, how much impact does
> > direct mapping fragmentation have on performance, in your case and in
> > general?
> >
> > In your case, I guess the performance gain is due to code gets packed
> > together and iTLB gets reduced. When code are a lot, packing them
> > together as a hugepage is a further gain. In the meantime, direct
> > mapping split (or not) seems to be a side effect of this packing, but it
> > doesn't have a direct impact on performance.
> >
> > One thing I can imagine is, when an area of direct mapping gets splited
> > due to permission reason, when that reason is gone(like module unload
> > or bpf code unload), those areas will remain fragmented and that can
> > cause later operations that touch these same areas using more dTLBs
> > and that can be bad for performance, but it's hard to say how much
> > impact this can cause though.
>
> Yes, we have data showing the direct mapping remaining fragmented
> can cause non-trivial performance degradation. For our web workload,
> the difference is in the order of 1%.
Many thanks for the info, really appreciate it.
Regards,
Aaron