Consider expanding number formatting cache to larger values

[stephentoub]

Several releases ago, we added to number formatting a cache of strings for the values 0 through 9:

runtime/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs

Line 269 in 20d4e30

private static readonly string[] s_singleDigitStringCache = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };

Since with the default format non-negative values aren't impacted by culture, this cache is used for all numerical types for all single digit values as part of value.ToString(). We should consider expanding the cache to larger values, e.g. up to 300 to account for other frequently used values on success paths, like HTTP success status codes.

The upside to this is significantly less allocation as well as better throughput for formatting such values. The primary downside is increased memory costs for a larger table, e.g. caching all values 0 through 299 would add ~12K in allocated objects (caching all through 1000 would add ~39K). If the full cost is deemed prohibitive, it could be made lazy, such that at the expense of an additional null check on each access, we could populate only the strings actually used. We could also clear the table under certain conditions if it was deemed an issue. And we could choose to not include the table in corelib builds where we expect memory to be at a premium.

cc: @marklio

Tagging subscribers to this area: @dotnet/area-system-runtime
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Several releases ago, we added to number formatting a cache of strings for the values 0 through 9:

runtime/src/libraries/System.Private.CoreLib/src/System/Number.Formatting.cs

Line 269 in 20d4e30

private static readonly string[] s_singleDigitStringCache = { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9" };

Since with the default format non-negative values aren't impacted by culture, this cache is used for all numerical types for all single digit values as part of value.ToString(). We should consider expanding the cache to larger values, e.g. up to 300 to account for other frequently used values on success paths, like HTTP success status codes.

The upside to this is significantly less allocation as well as better throughput for formatting such values. The primary downside is increased memory costs for a larger table, e.g. caching all values 0 through 299 would add ~12K in allocated objects (caching all through 1000 would add ~39K). If the full cost is deemed prohibitive, it could be made lazy, such that at the expense of an additional null check on each access, we could populate only the strings actually used. We could also clear the table under certain conditions if it was deemed an issue. And we could choose to not include the table in corelib builds where we expect memory to be at a premium.

cc: @marklio

Author:	stephentoub
Assignees:	-
Labels:	area-System.Runtime, tenet-performance
Milestone:	8.0.0

[jkotas]

caching all through 1000 would add ~39K

This math looks off. You need an array holding the string objects (1000 * pointer size) = 80kB on 64-bit. And you need to account for minimal object size (1000 * 24) = 240kB on 64-bit CoreCLR. The total size of this cache would be 320kB.

If the full cost is deemed prohibitive, it could be made lazy

Yes, I think it should be definitely lazy.

[stephentoub]

(1000 * pointer size) = 80kB

1000 * 8 bytes is 8,000 bytes, not 80,000 bytes. Have I misunderstood?

This wasn't actually based on math but rather on:

while (true)
{
    long m = GC.GetAllocatedBytesForCurrentThread();
    string[] values = new string[1000];
    for (int i = 0; i < values.Length; i++) values[i] = i.ToString();
    Console.WriteLine((GC.GetAllocatedBytesForCurrentThread() - m) / 1024);
    GC.KeepAlive(values);
}

[jkotas]

You are right, I got an extra zero somewhere. It is ~39k as you have said.

[stephentoub]

I think it would end up looking something like this:
main...stephentoub:runtime:smallnumberscache

[HideColumns("Job", "Error", "StdDev", "RatioSD")]
[MemoryDiagnoser(displayGenColumns: false)]
public class Bytes
{
    [Params(123)]
    public byte Value { get; set; }

    [Benchmark]
    public string ValueToString() => Value.ToString();
}

[HideColumns("Job", "Error", "StdDev", "RatioSD")]
[MemoryDiagnoser(displayGenColumns: false)]
public class Integers
{
    [Params(1, 12, 123, 1234, 12345)]
    public int Value { get; set; }

    [Benchmark]
    public string ValueToString() => Value.ToString();
}

Type	Method	Toolchain	Value	Mean	Median	Ratio	Allocated	Alloc Ratio
Integers	ValueToString	\main\corerun.exe	1	1.393 ns	1.392 ns	1.00	-	NA
Integers	ValueToString	\pr\corerun.exe	1	2.344 ns	2.335 ns	1.68	-	NA
Integers	ValueToString	\main\corerun.exe	12	10.003 ns	9.952 ns	1.00	32 B	1.00
Integers	ValueToString	\pr\corerun.exe	12	2.351 ns	2.318 ns	0.24	-	0.00
Bytes	ValueToString	\main\corerun.exe	123	11.340 ns	11.200 ns	1.00	32 B	1.00
Bytes	ValueToString	\pr\corerun.exe	123	1.183 ns	1.185 ns	0.10	-	0.00
Integers	ValueToString	\main\corerun.exe	123	13.434 ns	11.441 ns	1.00	32 B	1.00
Integers	ValueToString	\pr\corerun.exe	123	2.191 ns	2.181 ns	0.11	-	0.00
Integers	ValueToString	\main\corerun.exe	1234	11.536 ns	11.439 ns	1.00	32 B	1.00
Integers	ValueToString	\pr\corerun.exe	1234	10.444 ns	10.339 ns	0.91	32 B	1.00
Integers	ValueToString	\main\corerun.exe	12345	11.341 ns	11.260 ns	1.00	32 B	1.00
Integers	ValueToString	\pr\corerun.exe	12345	11.300 ns	11.150 ns	1.00	32 B	1.00

[EgorBo]

👍

from a memory dump of bingsnr (string objects - number of them in the current snapshot). But I can't say if they come from this formatting.

[stephentoub]

@EgorBo shared with me the dump he was looking at, and I did some querying of it using clrmd.

It has 8.7M strings, of which 177K are parseable with long.TryParse(s, NumberStyles.None, CultureInfo.InvariantCulture, out long value), and of those, ~25% are for values < 300. Of the 5.2MB consumed by all the number strings, ~1.2MB are consumed by these strings for values <300.

If I increase the threshold from 300 to 1000, that ~25% goes up to ~40%.

[geeknoid]

Can the size of the cache be determined at startup based on some environmental state?

Clearly, the cache memory is a trivial cost for a service application so in that environment it is warranted to make it large. But for constrained environments, the extra memory may be problematic.